The creatures from
Chapter 2 of the Planewalker.com releases would be the most common in Sigil, I think, if only because they're the most iconic.
Here's a non-racially specific table originally from J. Wright's now-defunct website:
The racial encounter table would be something like this, on a d20:
1-8: Humanoids and planetouched
9-12: Animals and pests
13: Monsters
14: Undead
15: Fiends
16: Celestials
17: Cordants
18-19: Elementals
20: Exotic Sentients
First, humanoids and planetouched:
Most common: Humans, the default planewalking humanoid. Humans often mate with other species, but any single hybrid race is going to be somewhat rare.
Planetouched: Aasimar, tiefling, chaond, tuladhara, zenythri/mechanatrix, shyft, shadowswyft, gloaming, phoera, genasi, mephling, feytouched, krinth, etc.
Other common races: Githzerai, githyanki, bariaur, khaasta, nathri, shad
Rare races: Buommans, elves, dwarves, fensir, gnomes, halflings, illumians, neraphim, orcs, spikers, wildrens, spellscales, mongrelfolk, half-orcs, goblins, kobolds and xill.
Very rare races: Bladelings, dromites, duergar, elans, half-giants, thri-kreen, kenku, maenads, mongrelfolk, xephs, changelings, nezumi, korobokuru, spirit folk, vanara, shifters, kalashtar, inspired, warforged, goliaths, aarakocras, raptorans, catfolk, lupins, diaboli, killorens, centaurs, gnolls, dopplegangers, half-ogres, underfolk, sharakim, sea-kin, dragonborn, half-dragons, azurins, dusklings, rilkans, skarns.
Animals and pests include things like pigeons, large gray "executioner ravens," rot grub swarms, humanoid immigrants from the Material Plane who have degenerated to an animal state due to the stress of their new environment, two-headed Aoskian hounds, Arcadian ponies, cranium rat swarms, giant rats, moon rats, ash rats, elemental vermin, fhorges, bonespears, rasts, ravids. ethyks, gathras and other livestock, creatures corrupted by magical pollution, lost pets, and more. Of course, there will be animals with the fiendish, celestial, axiomatic, anarchic, vivacious, elemental, and entropic templates.
Undead include both creatures of the undead type and other beings who can be easily lumped in with them: wights, ghouls, zombies and skeletons created by the Dustmen faction, vampires and liches living openly or in secret, spectres, ghosts, and wraiths, devourers, visages, atropal scions, entropic reapers, brains in jars, bhuts, allips, hooded pupils, grimweirds, quth-maren, alhoons, cursts, necropolitans, eyes of fear and flame, living creatures who are somehow half-undead, lepers and other living victims of piteous planar diseases, petitioners (lost, refugees, or here on a mission), worms that walk, rot reavers, and ordinary corpses. The authorities of Sigil don't tolerate undead who roam around killing people, but they may tolerate those who behave themselves. The other kind - pure monsters who need to be destroyed - end up in Sigil, too, of course, and cause trouble before someone finally hunts them down. Some may lurk in secret or beneath the city for years or centuries.
Celestials include angels, archons, eladrins, lillends, guardinals, moon dogs, couatls, quesar, ki-rin, opinicuses, hollyphants, lammasu, phoenixes, aleaxes, and risen fiends.
Fiends include night hags, yugoloths, rakshasas, fallen celestials, baatezu, tanar'ri, demodands, marrashi, imps, retrievers, barghests, kytons, and kythons.
Cordants include slaadi, formians, modrons, inevitables, Limbo stalkers, concordant killers, justicators, rilmani, visilights, and maugs.
Elementals include elementals, paraelementals, genies, lumi, sylphs, grues, azers, salamanders, xorns, tsnng, and so on.
Exotic sentients include things like spellweavers, illithids, beholders, grell, tsochar, psurlons, witchknives, cloakers, nagas, yuan-ti, neogi, neh-thalggu, zodars, skindancers, ethergaunts, shadar-kai, nerras, kaorti, zerns, phasms, nilshai, daelkyr, protean scourges, keepers, and other beings who don't easily fit any other catagory.