Guardian was awesome, both on the offensive and supporting my allies. I played Elementalist, Thief, Warrior and Guardian, and somehow my Guardian had the highest damage output (wut?). I also love the 5th greatsword skill (pull enemies towards you).
Elementalist was more fun than I thought, but unlocking all skills on the starter weapon took me until level 10, which is WAY too long, because I could only use scepter-dagger effectively at that level. That means unlocking all skills on an Elementalist would take ages, while on a guardian I had unlocked all weapons my level 8.
Warrior disappointed me a bit. I had been hoping for a really cool sword-sword 'hard hitting assassin' type of gameplay, but axes-axes was pretty cool.
I didn't try a lot of the Thief class, but I found the initiative mechanic irritating (not because it's bad, but because I had gotten too used to cooldowns being the only resource needing management after all those hours of playing G/W/E).