These are species with at least one recorded territory in Singapore, not a national checklist: a bird list from eBird or Avibase is more complete. The counts follow recording effort as much as wildlife, so a country with more visitors and more naturalists will show more of both.
The species with the most open occurrence records in Singapore, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Acridotheres javanicus

Pycnonotus goiavier

Oriolus chinensis

Treron vernans

Spilopelia chinensis

Aplonis panayensis

Cinnyris jugularis

Corvus splendens

Todiramphus chloris

Amaurornis phoenicurus

Eudynamys scolopaceus

Hirundo javanica
The reference database has no regions filed under Singapore.
The field guide covers Singapore on the map, but none of its species have an entry written for them yet.