The best dollar/hash ratio seems to be the Radeon 5850/5830 GPU's. They are capable of 300-350+ Mh/s and can be found on Ebay for around a hundred bucks each. By my calculations, two of them should net about .05 BTC per day - about $5 per day at the current exchange rate.

https://en.bitcoin.it/wiki/Mining_hardware_comparison

Two 5850's are around 300 watts. That's 7.2 KWh per day. At 18 cents per KWh, that's an energy cost of about $1.30 per day.  $3.70 per day net will pay off $200 cost in under two months - assuming a relatively stable exchange rate, of course - and there's the rub! The exchange rate could go much higher, or it could drop back down. That anybody's guess. You also have to factor in the difficulty rate. As network hash rate rises, your return drops. A lot of folks keep saying all these ASICs are going to come on line, but I have my doubts that its going to happen quickly. The BFL ASIC seems to be classic vaporware with no materialization in sight, and some people saying its an outright scam. If it is, its a very elaborate 'long con' type of scam. Only time will tell. The Avalon ASICs are coming out, but in pretty low numbers. IMO not enough to seriously effect the hash rate for some time (years end maybe).