Dura Ace cogs deliver some serious bang for the buck, their quality is very high and the price is far less than many boutique cogs.

Surly cogs are also good (and have gotten better) but they are damn pricey.

I have been running some Formula cogs for quiet some time and they have been performing really well and they cost < $10.00.

My favourite cogs (1/8 only) cost me 0.00 but getting those requires one to have a parts bin full of old coaster hubs with their beautifully cast threaded cogs and lockrings... (modern coaster hubs have a splined cog).