Thief, Engineer and Mesmer. I think the Ranger is beginning to grow on me as well, but it's still too early to tell...
Thief, because of its nimbleness and fast paced combat. I love the Initiative system as I don't feel restricted to CDs and try to use it wisely, though more often than not you can get by by DBing the enemy to death

I can take on impossible odds and survive but I can die just as easily if I don't pay attention. And I love the stealth mechanic! It doesn't have the "versatile" feeling I get with my Engi, nor the nuanced, subtle yet flamboyant combat of the Mes - but what it does, it does with style, and does it well.
I'd advise people who want to play a thief to play as dynamically as possible, and work on both their proactive and reactive skills as players - and to pay close attention to each mob, learn which attacks to be most mindful of, and learn when to use their abilities to greatest effect. You can tear the faces off of four same-level Ettins at the same time if you pay attention to the fight.
Engineer, because of the sheer amount of delightful toys it has at its disposal! Its versatility is amazing, and the sheer amount of interesting options it has keeps it fresh. Though - I believe it truly shines in a group, and played with a more team oriented, supporting mentality (even though it's still a good solo class).
I'd say that people who want to play an Engi should keep an open mind, try out new things and keep experimenting! There are tons of options for this class, don't be afraid to try them all!
Mesmer, because of its unique feel and comparatively steep learning curve. It's not easy at lower levels (my Mes is only level 13), but I'm finding that I can pull off really crazy stuff if I play flawlessly. There's a lot to manage at the same time - enemies, cooldowns, shatters, illusions - but it's all the more rewarding to play with for that!
I'm a complete newb to the Mes, but so far my advice would be to control the fight. Plan ahead but have a backup in mind for when things go awry. Be alert at all times, learn your abilities inside out, learn how to time your shatters and when to better use them (they can really turn the tide of any fight either way!) - and be persistent. It's incredibly rewarding to play!
As for the Ranger, I just started one but it's growing on me. Mostly because it feels quite relaxed to play, with the pet tanking things are easy most of the time. However there is still a lot of depth to the class for those who would master it, with proper positioning, situation assessment and pet management being very rewarding. It's very newb friendly, but has a ton of nuances for more experienced players to discover.
My 2 cents