Weekdays mornings: NO queues
other times: MRT Stations, Shopping Centres, CBD (Chinatown, Orchard, Marina), Shenton Way area.
Saturday nights: Clark Quay night spots, Orchard Rd (but i always see many kosong taxis), Airport.
Changi Airport i heard cabby gota wait for 2 hours to get to the customer.. one cabby even told me when a BIG aeroplane comes in, all the taxis in the queue also wont be enough for that many passengers

i notice some taxis dont like to queue at neighbourhood MRT stations because ppl who board from there usually goes/stay very near, within the neighbourhood.. they rather go to city centre
Still, worst airport queues usually at night. Also, seems during day time. quite few taxis and customers... maybe all took bus or MRT?
additional note: Places which taxis likes to cruise around: Light Industrial Estates, one-lane roads, lorongs, drives, streets etc.

sad.. my place, surrounded by AMK ave 5 and YCK Rd, sure very hard to flag
down taxi..