In order of increasing fun, I leveled Loki, Nightcrawler, and Hawkeye from 50 -> 60 this week as well.
Loki has some interesting design elements, but the net experience is the most miserable on my roster. He has literally zero single target damage skills. I chose to focus on a sorcery build, because the ice tree seems to lack coherence and the clone tree is a pale shadow of GW2's mesmer. Sorcery is fine for mobs
but otherwise it's a rote and awkward rotation for mediocre damage.
Nightcrawler is one of my favorite X-men, but I'm not a fan of his hit-and-run playstyle. I refuse to use swords out of principle, but you can actually pull off a solid build with just the other two trees. He felt better in the 50-60 range then earlier in the level curve, but still meh.
Hawkeye was surprisingly the most fun of this sorry lot. This was mostly due to one underrated skill- Freeze Arrow. I understand CC is problematic in this game, but freezing works. You can AoE freeze mobs for over 10 seconds each. You can consistently freeze bosses- not necessarily on the first hit, but you can just spam Freeze Arrow until it activates and then follow up with your AoE DoTs. He's also got several other nice touches: a viable basic, a single target that you can properly time to weave into rotations, a roll that stuns, a jump-back, and an AoE toughness and constitution buff. This last skill, the Medical Arrow, actually pumps out some nice support.
One commonality among this group: all bleed spirit, such that I was compelled to give each my stack of Gibborim as well as slot +spirit gear.
I'm looking forward to not playing any of them again, but if forced to stick with one right now, it would be Hawkeye.