I highly recommend you grab Singed. He's a bit tricky to get started with, but he's only counterable by ranged tops. (swain, AD carry, vlad,...) however, even a ranged won't stop Singed from getting his farm, so you'll get a good late-game anyway. If you spend some time learning to play him, he's one of the most safe solo top's i can give you. You've got voli is top, wich is a good choice, but the enemy will grab ignite or a healing-reducing champ and faceroll you. Kat is good as well, but most tops have CC, wich is her demise. Rumble is REALY good, almost as good as Singed, (in getting last hits and just general surviving) but Singed is better for when you realy need a TANK, not some half-arsed offtank. Not to say rumble is bad, but he doesn't fit well in teams with for examlpe shaco in the jungle. As for GP, he's a very good solo top. Can't say a lot of bad about him, but I feel like Singed is better for the team. GP has his ult, wich can turn teamfights around, and he's a good split-pusher, but so is Singed.
TL/DR: Singed rules, get him.