Which lane does Gragas best fit in?

The jungle lane.

I like Gragas at top lane because he can land his poke better and does not need to build full ap to compete with the ap mages.

I don't think he is very effective in the mid lane anymore. Top and jungle seem to work OK for him now. I've also seem some decent gragas supports too.

Saw someone try Gragas mid. They hadn't played LoL for a few months, so they thought he was still OP as hell. They got wrecked so hard xD

Only seen Gragas do good post-nerf in top lane or in jungle. Pretty sure he was redesigned for those two roles.