First game was a Zelda clone with Final Fantasy trappings and a Green Aesop plot/ending. It's also one of the best games on the Original GameBoy.
Second game was Secret which was a great looking (at the time) ARPG with a great soundtrack and multiplayer, which at the time, was an impressive feat. Nowadays, you can easily find games that do the above and more.
Third game was SD3 which has no proper western Mana designation yet and Squeenix continues to refuse to bring this over officially. Otherwise, its the game that really set the tone for the series and is generally an improvement over Secret (although I don't think they fully replicated the multiplayer aspect, only allowing for two players instead of three; the environments/dungeons are generally less inspired (not that they were that much better in Secret); and the soundtrack isn't quite as good).
Fourth game was basically a SaGa game if SaGa games were sidescrolling beat-em-ups with the usual Mana trappings. It does some things really well, but the game has some serious balance issues and kinda feels half-baked in spots. Still a solid game though.
Fifth game is Sword. It tries to be all four of the previous game, but the fact that the balance is awful and that its a remake of the first game that doesn't really add anything to it really drags it down. And being on the GBA really doesn't do its soundtrack any favors.
The rest are hot garbage (but have good soundtracks at least).