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My favorite leveling experience in an MMO, for the first character, was SWTOR. If they had incorporated the questing system of GW2 it would have been even better.
I really enjoyed leveling in classic WOW. At this point it's terribly dated but at the time I loved it.
GW2 is great to explore in, but it's just not nearly as addicting at the moment. It definitely feel more grindy to me than the first two.