Since you asked for an opinion - I'll give you one.
I think the Necro is more interesting to play - lots of versatility but it is a matter of doing AE's and watching multiple foes die as you evade, dodge, and run around leading them to their death. In PvE you can use pets and almost never get hit - but if your pets die - you can be in a world of pain yourself and there is no telling if you can get them back in the fight quick enough.
If you become the master of Wells and Marks - you'll more than likely never use a pet. - You lay down marks and then lead your victims through them - then hit the Wells and watch them melt - switching it up with a variety of weapons you can mix to gain health, do retaliation, DD, siphon health, or more AE's.
Mesmer is trickier - they aren't easy to level but can be a great support class and you can pick a variety of playstyles with them also. There are no big flashy spells - but they have some fairly good DD but do lack AE unless you learn to functionally shatter your illusions and clones - the shatter build as they call it. This can do a lot by adding condions - albeit random in aspect - as well as their AE damage to foes near your target.
They have high survivability if you focus on the Glamor line with stealth abilities - which also detarget you and regardless of what people say about only fools can't tell the real mesmer from their clones - when they steal away into the abyss - it is only the fool of a mesmer that doesn't get lost in the shell game. Especially in a crowded WvWvW environment. I think of the three - they are the most challenging.
The Guardian - well - they have high survivability - and can be great in PvE and hard to kill in WvWvW. But as with all classes - when you get focussed by 5 other players you are probably going to die - especially if they know enough to rip away your boons.
I leveled 5 characters at first to 10 - then to 20 - then 4 to 40 - of all the ones I leveled the Necro was the first to 80, then the Engineer, and my ranger is now at 77. I have an Ele at 25 now and I started reworking the Mesmer - who I have deleted and redone several times. I also deleted a level 40 thief to try others since I'm not mush into melee.
One thing I will say is beware... several people who leveled heavy armor characters say that they spoil you as the clothies are just so squishy it becomes frustrating to level them. But, I will also say - that once you level a clothy - leveling a heavy armor wearer can be a little boring - clothies keep you on your toes a lot more while you avoid death.
So for me - it would be Necro, Guardian, Mesmer.... Although Guardian just wasn't my playstyle and at lvl 20 they were delelted to try Warrior :) Above all else - have fun.
» Edited on: 2013-02-01 14:47:29