The Daily Grind: Which MMORPG offers the best leveling curve?

Last weekend I logged into a City of Heroes rogue server to take my kids for a round of radio missions. I let my son look through my characters first so he could decide which toon I’d bring with him (we wound up with two Masterminds, which was awesome), and he remarked on the fact that I have so many characters in the middle level-band… whereupon I launched into an explanation for leveling curves in MMOs.
See, City of Heroes’ leveling curve is rough, even on rogue servers with double experience. Like a lot of older MMORPGs, it allows you to level pretty quickly early on, but unless you’re cheese-farming some AE missions, the late-game leveling slows waaaaaay down for people playing as intended, as each level takes significantly longer than the one before.