I don't think anyone asked for this, but I'mma do it anyway. Monsoon breakdown of the entire Ratchet & Clank series:
Foreward: Ratchet & Clank is my favorite PS2 series. Each of the four games had a clear direction, made specific improvements and further experimented with the series' formula with each sequel, and was a consistently fun blend of running, jumping, and shooting with ridiculous equipment and weapons, plus several alternative gameplay modes in each game like hacking minigames, racing, turret shooting, space shooting, and arena battles. The stories of Ratchet games are character driven and HILARIOUS, with some chuckle-worthy jokes in almost every cutscene. Each game is maybe 10-14 hours to finish, with a larger time investment if you get into item collection or special challenges in a big way.
The PS2 games are a clear "original trilogy" plus a fourth spinoff (no "Clank" in the title, scaled down stages, etc.), and I really enjoyed all four. The recent PS3 collection is the first three games. In the PS3 era, Insomniac Games made two full-blown Ratchet & Clank Future games, a third half-game taking place in between the two Future titles, and then two more recent spinoffs that introduce new gameplay types, including co-op. There are also three handheld R&C games, and they're decent, but I don't consider them part of the Ratchet canon as they weren't made by Insomniac. I'll be using the American titles for each of the games and rating them each with up to five stars of quality. Five stars = excellent, four stars = pretty good, three stars = decent, two stars = sub-par, one star = bad.
Ratchet & Clank (PS2, 2002)
It's hard to go back to the original after playing the recent games, because the targeting mechanics are unrefined, the platforming is less sticky, and there is no weapon upgrade system to speak of. Still good, though, with a well-crafted storyline and some truly ingenious platforming segments. The old InfoBot macguffins were nice, and I sort of wish the series had continued using them in some capacity. The first Ratchet & Clank is an important part of the series to introduce the main characters (mostly Ratchet, Clank, Qwark, and The Plumber) and to just be a good foundation, but I prefer a number of its sequels. Morph weapon turns foes into chickens. Four stars
Ratchet & Clank: Going Commando (PS2, 2003)
I think Going Commando is where the series finds its groove. The expanded arsenal is upgradable via EXP for the first time, the stages are larger and more diverse, the number of collectibles is huge, and we see some good character development out of Ratchet (who settles into a more laid-back persona) and Qwark (who is, for my money, the best character in the series). The villains of Going Commando are probably the best in the series, and Ratchet has a voice actor change from Mikey Kelly (Michelangelo in the old TMNT cartoon) to James Arnold Taylor (Tidus in Final Fantasy X) that proves lasting. Oh, for the record, Clank has always been voiced by David Kaye and Qwark has always been voiced by Jim Ward; they're awesome. Morph weapon turns foes into sheep. Five stars
Ratchet & Clank: Up Your Arsenal (PS2, 2004)
My second-favorite game in the series. I think its storyline is roughly as good as the first two games, but Qwark is at his absolute best and the game also introduces Dr. Nefarious, who becomes a major character in later games as well. UYA has my favorite weapon selection in the entire series, and some awesome in-game bonuses from having completed save files of the first two games. Great stages and collectibles, but fewer than Going Commando and generally shorter stages. Weapon EXP is better than Going Commando, with more upgrade tiers and better weapon mods. The alternative gameplay modes (Rangers, vid-comics, arena, racing) are probably my favorite in the entire series. Sadly, Up Your Arsenal is the last Ratchet game to feature voice work from Neil Flynn. Morph weapon turns foes into ducks. Five stars
Ratchet: Deadlocked (PS2, 2005)
Spin-off game whose gameplay consists of ENTIRELY arena stages. Sure, the arenas are a good size and the boss battles are solid, but the smaller arsenal (only 10 weapons, with one of them endgame-only) and lack of stage variety is a major downside. The highlight is probably weapon customization, which improves upon the mod systems of both Going Commando and Up Your Arsenal. Still, Deadlocked is a letdown compared to the original trilogy. I think a lot of development time was spent on the multiplayer functionality, which was okay but unbalanced. Morph mod turns foes into chickens, sheep, ducks, and pigs. Three stars
Ratchet & Clank Future: Tools of Destruction (PS3, 2007)
The older Ratchet games are crisp and nice to look at. The Future trilogy is freaking GORGEOUS. This was the game that got me sold on the visuals of the new generation of consoles. But as a Ratchet game, Tools isn't in the top tier. The weapons are great, but the new characters aren't quite as interesting as older NPCs and the Clank-only missions are weaker than usual. The addition of Combat Devices is unnecessary (basically makes ammo more expensive for a few weapons), but the Raritanium weapon mods are a great change to weapon upgrades and the weapon selection is quite strong, maybe better than Crack. Firefights are still pretty good, collectibles are still pretty good, the lack of PSN trophis is disappointing. The Six-Axis controls (used on a few weapons and transportation modes) are okay, but ultimately I wish Insomniac implemented them less frequently. Plot cliffhanger at the end was extremely frustrating at the time. Morph device turns foes into penguins. Four stars
Ratchet & Clank Future: Quest for Booty (PSN, 2008)
The download-only game is probably not worth the $15 price tag, but is decent fun for its 3 or 4 levels and provides a crucial plot point to serve as a bridge between Tools and Crack. Your arsenal is five or six re-used weapons from Tools of Destruction and the gameplay is good, but nothing to write home about. My recommendation: play Tools of Destruction, read a synopsis of Quest for Booty, then go straight on to Crack in Time. No morph weapon. Two stars
Ratchet & Clank Future: A Crack in Time (PS3, 2009)
This is what I'm talking about. A Crack in Time (the original working title "Clock Blockers" was nixed by Sony and I still wish they hadn't) has bigger and better stages and some fabulous firefights and arenas. Space shooting and space travel is the best in the series, and the Clank-only segments (mostly-forgettable parts of earlier games) are the best puzzles in the series. They ditch the Combat Devices from Tools of Destruction, make the best of the old Devices into new weapons, and bring some old and new weapons to the table to make a solid arsenal overall. The storyline is excellent, delving into the pasts of both Ratchet and Clank in a big way, bringing Clank to the story forefront for the first time since the first game. Welcome returns of several old favorites as well. My favorite game in the series, but if you like Commando or Arsenal better I won't judge you one bit. It's a close call. Morph weapon turns foes into chimpanzees. Five stars
Ratchet & Clank: All 4 One (PS3, 2011)
This is pretty good as a co-op action game, but a little weak in single-player. Players control Ratchet, Clank, Qwark, or Nefarious, and each can use a unique gadget in addition to a modest selection of traditional Ratchet weapons. Stages are more linear than usual, but boss battles are big and entertaining and the shooting is still fun. Great in a group, much less-great alone. I'd say more, but I never finished All 4 One and won't be inclined to pick it up again unless I can get multiplayer going easily. Morph weapon turns foes into pigs, sheep, or dinosaurs. Three stars
Ratchet & Clank: Full Frontal Assault (PS3, 2012)
Probably my least-favorite game in the series - not a very awesome way to celebrate a tenth anniversary. FFA was released just last week for only $20, but it also has only five stages and they balance real-time strategy with platform shooting in a way that's not always fun. Not very many weapons either, with only a few new additions to a selection drawn from the other PS3 games. Challenge level is also quite low in a series where it's never been that difficult to begin with. The co-op is better than the single-player when I tried it, but only made the game easier and seem even shorter. Look, my favorite parts of Ratchet games are the awesome weapons, funny characters, and massive variety of gameplay, and FFA falls a little flat in each of those elements. The paid-DLC skin packs don't enhance the experience, either. Take what I say with a grain of salt, though - I only played it for maybe 3-4 hours and I didn't even find the morph weapon. Two stars
Look, long story short (too late), I'd go with the Ratchet & Clank Collection. PS2 copies of the first four games are probably five bucks a pop if you don't mind tracking them down. If you like those, then by all means continue with the newer PS3 games. The Future trilogy heavily relies on background from the PS2 trilogy, and Crack is a direct continuation of Tools. You can totally skip Deadlocked, the PSP games, All 4 One, and FFA and not miss anything important, but you really ought to try the first three.