IT'S PTOLEMY
Of course it's Ptolemy. Ptolemy's my favourite character.
Let me give you a long list explaining why it's Ptolemy.
1) He went to the Other Place with Barti
2) He gave Barti the three gifts
3) Heck, he TRUSTED Barti (that's one of the gifts, but I'm pointing it out anyway)
4) He had the power to stop Barti swearing (too much) in front of him, which just points out how important he was.
5) Ptolemy's great, so there.
6) He actually cared about what Bartimaeus thought, unlike Nat, who only cared what Jane thought (that's a tribute to Luciene. He cared what the PM thought too, though I'm not getting further into that)
Haha, it's Ptolemy, end of story.
Sorry about the rant.