I've never played Wild Arms 1
I loved Wild Arms 2 though, but due to a stupid rage during a breakup, deleted my save file not far from the end. Great characters and storyline, great villains.
Wild Arms 3 was actually a bit underwhelming compared to 2. I got annoyed with Virginia's daddy issues after a while, the overworld is bland as hell, and it kinda drags later in the game. I was probably 2/3rds the way through when I stopped. It's the most "wild west" feeling of the series.
Wild Arms 4 is a totally different animal. It doesn't feel anything like a Wild Arms game. Everything is different. World Map exploration is gone, it often becomes a side-scroller RPG with platforming, the battle system is a tRPG hybrid, and the Wild West theme is almost completely gone. Never-the-less I really enjoyed it, and some of the characters (Raquel) are absolutely amazing. I stopped JUST before the final boss, and I've always regretted it. One day, I WILL FINISH IT, but it's been years. I would say that 4 is better than 3, but is like a completely different series. Where I thought that 3 was a boring rehash of elements fine-tuned by 1 and 2... 4 is fresh, but a little uneven at times.
I've heard bad things about 5, never played it.