I think you can do well with any of those classes. Which is best for you really depends on what content you’re interested in and how you like to play. To put it in general terms:
Thief plays FAST. You deal high bursts of damage and use frequent evasion, leaps, stealth, and/or teleports to move around the field and avoid damage. The “rotations” are among the most straightforward of any class, but thief is played with a lot more movement than other classes. It’s also less forgiving.
You have the tools to survive as well or better than any class in many scenarios, but you don’t have the health or armor to take a lot of hits if you make mistakes. So although the playstyle is fairly simple, the speed at which the class is played may force some additional errors if you aren’t comfortable with it.
Ele is another high damage class with a fast playstyle, but for different reasons. Elementalists essentially have twice as many abilities to work with as thief, even though they don’t have a combat weapon swap. To play this class well you need to know all of your abilities, when to use them, and be quick enough on the draw to execute when it counts. Being a little sluggish on ele will cost you just like it will on thief, but you have a lot more to learn before you get it down.
Guardian is a class I haven’t tried yet. From what I hear, it has solid healing and group support as well as access to damage mitigation (aegis, protection, etc.). I get the impression that it’s a slower, tankier class than thief or ele. It might make a better option for players easing back into the game. But that’s just speculation.
