My first profession in Guild Wars was mesmer, and it is still my favorite. I was initially drawn to it by the aesthetics of the female mesmer and my best friend's description of it as a mental-powered character class.
But mesmers are very hard to play,* and I was crazy to start with one. (Of course, I didn't know any better, and neither did my friend, since she had only owned the game for two days at the time!)
It's rather versatile, but it's especially good against any caster profession. They can do shutdown, e denial, and interrupt especially well, and can do conditions, too.
I find that mesmers are heavily under-appreciated. PvP players, especially those who do a lot of Heroes Ascent, tend to have a better understanding of what they can do. When I see someone in local chat saying, "Mesmers are for noobs," or the like, I know they have no clue what they're talking about.

And when a team in HA tries to take me out immediately when I play a mesmer, I know they
do, and we're in for a tough fight!
*Note that I make no claims as to my actual skill…