I actually deleted my original character, which was a Titan maxed out on both Striker and Defender. I'm now using:

-Level 27/28 Titan Female Striker (just maxed out Striker so going to work on Defender as it's better for PVE)
-Level 23 Female Hunter Gunslinger
-Level 22 Female Warlock Sunsinger

I had a level 28 Male Titan that I initially created but I ended up deleting him. I use the Titan for 99% of my time playing but the other classes are cool, too.

I'd say the Hunter is probably the best for the Crucible simply because both of the Hunter classes have really overpowered Supers. Gunslinger is basically a guaranteed 3 kills if you can aim (4 if you have the Exotic helmet that gives you an extra shot) and Bladedancer is the most overpowered super in the entire game, IMO.

I've spent the least amount of time on my Warlock, however, it's probably up there with Titan Defender for most useful during PVE because of it's super being able to self revive, which is just indescribably good during things like Nightfall or Raids.

I'd like to get all 3 around 27 or 28 so that I can do the weekly heroic and get 9 strange coins for each of them, guaranteeing me at least 27 per week. It'll be awhile, but I'll get there eventually.