I have used many different ones and I have the better results with the BG 44K. It cleans more of the intake and exhaust path as well as a fair cleaning of the O2 sensor and catalytic. The 44K is about $20 retail so it is not cheap but I haven't found cheaper products as effective. The premium fuel maximizes the effectiveness and I always recommend changing the filter because it cleans gunk out of the tank too.
I am not totally stuck on the BG products...
The Valvoline SynPower Performance System (3-part) works very well
http://www.valvoline.com/pages/produ...asp?product=56
And the Valvoline SynPower Complete Fuel System works pretty well also.
http://www.valvoline.com/pages/produ...asp?product=54
I have found the Greased Lightning Fuel System concentrate would be a good choice also since they are good cleaners and cost between $12 and $15.
http://www.greasedlightning.com/cont...rodsection=eng
The key component to avoid is methanol (wood alcohol) but ethanol (grain alcohol) is ok but alcohol-based products don't clean very well in my experience.
Troy