If you're just looking to play a clothy, for new players I would suggest a Necromancer first and an Elementalist second. Necromancers have the most base HP of any of the casters and are probably the easiest to talent for a new player. Elemenalists look pretty straight forward, and they are once you spend some time with them but figuring out attunement synergy takes a little more attention span than, say, a pet build Necromancer. They're also REALLY squishy unless you build them for survivability.
Mesmers IMO are the more complicated of the three and it took me a while to figure them out mechanically. So much so that I started mine on day one of the head start and played him to 37 that weekend then shelved him until I got more experience with the way the game works. I got two other characters to 80 before I picked him back up and felt comfortable with a couple of builds that I use regularly now.
Mesmers are the least played class for a reason. Obviously not everybody will have the same experience I did, but most would agree they're a pretty complicated class. Plus all the pink and butterflies tend to scare most men off.
All of that said, all of the professions in this game have several different builds that are all viable for you to play once you get used to them and do some research and thinking. A Necromancer could be a mini-zerg with a pet army or they could be a one-man debuff machine turning the tide of a bad fight with smart use of Wells and Marks. Elementalists can be long ranged support or up-close and personal assassins. My Mesmer? He's got ~23k HP and likes to run head-first into enemy garrisons in WvWvW.
I'm not so sure mesmers are the least played profession. Usually when people speak about least player professions I hear engineer and necro. Seems to me that I'm always seeing mesmers around, at least on my server.
I think of lot depends on whether your main focus is PVe or PvP. For example, mesmers in PvP are quite powerful. In WvW, elementalists are quite useful, as well.
Necro pet builds tend to be mostly useful in PVe and the only other build that seems to be useful is a condition wear them down build. That's why necros have so much more health than other caster professions. Because it takes longer to down things.
I'm going to say that it depends completely on your playstyle rather than just making a recommendation. Of all of them, I believe the ele requires the most skill to get right, because there are some pretty easy builds for the mesmer that take relatively little skill to master.
So, do you prefer more large direct damage, more condition damage, wearing down the enemy? More sleight of hand bedazzle and confuse? Those are really the questions.
In this game, it's not really about best. It's all about playstyle. Try one of each, make sure you unlock every weapon on each one and play up to level 20, then decide yourself. Because everyone is going to have a different answer.
I'm not so sure mesmers are the least played profession. Usually when people speak about least player professions I hear engineer and necro. .
Oh I was going on the last graph I saw they had released, looking back at the date now though it could be waaaaaay different now. I do see more Mesmers now than I used to but I still see way more of every other class but engineers unless I'm hanging out in the jumping puzzle in WvW. I think it's gotten to the point that people have gotten their main characters up and they're trying out other stuff. Most of the mesmers I see are in the first few zone tiers. I'm usually the only Mesmer among with a few warriors and thieves when I do pug dungeons. Now when I do guild runs there could be three or four of us in the run!
Those cookie cutter builds are pretty good but when you customize your character for your own preferences and play style it's so much better than any build anyone else could give you. The great part about this game is that any class can be picked up and played by anybody of whatever skill level, as long as you put a little effort into it.