HMmmm, I've found that just soloing a door mission you get, followed by selling your enhancements, and doing another one is really good XP actually, unless your char can't solo I guess...

Actually finishing missions can be decent xp, if you can really blow through "arrest 15 lost" missions fast, it adds up quick... although it's a fuckload faster / easier with a movement power to find your targets quick.
You're probably best off going to Perez Park (well at lowerish levels) and joining a big Hydra group where you're on the lower end of the level range.
Higher level chars get a (slightly?) bigger share of XP, but still I think it's pretty good if you're lower than they are - just, and this is key,
DO NOT let them sidekick you. Sidekicking is fine, but if you're tryin' to power level you'll get less xp sidekicking (not to mention lower level enhancements).
Mostly though the reason I say hydras in PP is your best shot is cuz you can join a group at like level 10+ and it's *constant* fighting cuz they respawn fast and they're worth a lot so it's just non-stop xp trickling in - it's a little tedious after an hour or so but you know that's powerleveling for yah. Your best off putting on your looking for team flag and doing a /req every 5 mins or so saying something like,
"level X, defender/controller/whatever, Looking for Hydra team."If you're under level 9 and you wanna power level, same as Hydras but go to PP in a group and walk up and down the streets blowing through groups of hellions and skulls. The mobs' levels start at like groups of 10 at like level 7 and works their way up to level 10s, blowing through these groups is damn good xp. And once you hit like level 10 and the xp isn't as good, you're ready for hydras.

If you're level 6 or less (can't get into PP) then you level so fast you should just STFU. Actually go to the PP entrance area in Atlas Park or Galaxy City. You'll find little groups of level 5s 'n 6s there, kiill 'em until you level to get from level 5 to 6, but just doing your door missions works pretty damn fine / fast at that level.
Just remember power leveling is kinda boring, just take an hour or two out and commit to wandering in circles in an appropriate area killing everything that moves (preferably in PP in a group with a healer so you never have to stop moving).