Skaven are, IMO without a doubt the best team.
They have the fastest player in the game (Gutter Runner) and their slowest player is faster than some teams fastest (unless you count Rat Ogres, which I don't, in which case Skavens slowest player is as fast as some other teams fastest).
Added to that, they have access to the weird and wonderful Physical Abilities and, unlike Elves, are cheap enough that you don't much care if one of them keels over and dies on you.
Wood Elves might be a faster team with both their Runners (MV:9) and Wardancers (MV:8), but they're too fragile for their point cost to be effective over the long term.
Once a Skaven team has a couple of Gutter Runners with Leap, they're almost unbeatable short of exceedingly bad luck. This I know from personal experience of playing such a team!