This question comes up fairly regularly. My favorite is unscented mineral spirits. It's cheap, it cleans better then anything that I've tried previously, the smell isn't overwhelming and it doesn't rust holes in metal parts washers. Anything water based I've found to be worse then using nothing at all since they usually initiate a coorosive reaction on the metal, and they turn a lot of the caked on stuff into something similar to concrete.
I did this transaxle case in my parts washer in 20 minutes with a brush and a small screw driver.
