Others have named some champs it's particularly good with (especially pushers for saving towers or knocking down enemy towers when the opportunity arises). But I find it's more about team composition. If the rest of my team isn't taking teleport, I might. If the rest of my team all have teleport, I'm far more likely to take another combat skill like Exhaust or Ignite, or even Clairvoyance or Fortify (spells it's nice to have on a team anyway).
If you are really thinking team-wide, though, I think you want ~2 teleports on the team, on support and pusher characters. You want your carries to take powerful spells for fighting, which teleport is not. Your solo mid Annie who wants to gank a lot will benefit more from Ignite or Exhaust, or maybe you throw Flash and Ghost on your physical carry for chasing. Or your carry is Teemo, who perhaps really wants Cleanse/Flash/Ghost for escaping (because he doesn't have a good escape ability otherwise) and Exhaust (since he has no stun, either). And Teemo is fast anyway, so why waste a slot on Teleport?
I don't think Teleport is essential on anyone, but it is essential to have on the team (especially if you don't have fortifies). You need some way to defend a distant tower from at least a mild attack.