All of them, with almost no exceptions. There should be Brobdignagian neighborhoods full of giants, as Ken Marable suggested, as well as Liliputian ones full of tiny creatures. Sigil is the gateway to a thousand worlds and planes, and there's no reason to put any restrictions on what can be found there. However, I would make planewalking races like githzerai, shadowswyfts, bariaurs, planetouched, humans, gloamings, illithids, angels, devils, slaadi and so forth more common than races common to only the Material Plane like elves, dwarves, gnomes, ropers, goliaths, and manticores.
One of the very few exceptions is that there are no mercanes in Sigil. According the PSMC2, they avoid the Cage like the plague. No one knows why, but they seem terrified of it.
The other exception, of course, is that the city will accept nothing with a divine rank, although there are rumors of something going on in a place called Harbinger House where that might be changing...
The "planar commitment" trait from the Manual of the Planes is without question the most pointless, moronic, and fun-destroying rule in the book. It deserves nothing but ridicule whenever it is mentioned. Petitioners will tend to remain in their final rewards/punishments by default, but there's absolutely no good reason to create a rule outright banning them from ever escaping their planes. Why shouldn't I be able to create an exceptional petitioner as a PC? Why can't I have a petitioner flee prosecution or a messy relationship in its home domain, or gotten lost due to an errant planar rift or portal, or been kidnapped, or immigrated with a vast wave of other migrants upon the death of their patron deity? So many story possibilities killed with one ill-thought-out line.
What's more, the rule ignored established lore, which said that Sigil does have petitioners in it - not a lot, but some. The Grixxit is a petitioner from Ysgard, formally a member of the Expansionist faction, bent on revenge for the destruction of her fellows. The Planescape boxed set included a short scenario involving a petitioner of the Chinese pantheon who accidently got rerouted to Sigil while Yen-Wang-Yeh was busy reporting to Shang-ti. There's another adventure that involves a petitioner from the Beastlands going to Sigil to complete some unfinished business from its life. The former factol of the Fraternity of Order, Hashkar, was even a petitioner.
If the idea of the trait wasn't stupid enough, the Manual of the Planes managed to make it even dumber by adding a bunch of arbitrary and unexplained exceptions. So lantern archons, lemures, larvae, and manes can leave their planes, but petitioners from Acheron, Ysgard, the Beastlands, and the Outlands can't? And we're given no justification for why certain kinds of petitioners are mysteriously immune to this otherwise immutable rule. For that matter, we aren't given any justification for why any petitioners should be bound to it. If some forms of petitioners can somehow avoid this restriction, why can't others?
Finally, as I said, the rule is pointless. Exactly how will the game be harmed by allowing petitioners to occasionally travel? There's no reason for such a restriction. Will not travel, sure, I can buy that. Most won't want to go anywhere, or their gods won't allow it. But can't, as a trait inherent to the template? Such nonsense.