I voted Necromancer because it has very versatile support skill trees. I'm running a magicka dps, which can also be an effective healer, and with a gear swap can also be a tank.

Not saying other classes aren't versatile, I actually multi-role a lot of my classes because I end up with too many skill points to spend. My stamina dps Warden also tanks and pvp, magicka Templar dps and heals, stamina dps Dragonknight tanks and pvps, and stamina dps Nightblade also tanks.

I like the classes, one can always find ways to make something work.

Warden without a doubt, since its the only class that can do a competetive Stamina-Healer. With the right Equipment and Setup you can do Tank, StaminaDPS and Healer without respeccing. No other class can do that.

Warden can do all roles without effort and has good skills for everything.

Templar - good healer and DD
DK - good tank and DD
Nightblade - good tank and DD

Necro - good DD semi decent tank but definitely not bis.
Sorc - good DD