There have been a number of discussions on Mechanics about best solvents. BF poster is a professional chemist, and he has posted a bunch of interesting facts about different solvents. Main conclusions I've come to from his and other chemists' postings are as follows.

1. Odorless mineral spirits is the ideal solvent because it (a) is fairly low in toxicity and (b) evaporates the quickest (good for keeping away rust, also).

2. Simple Green only works because of the polarity of water molecules. That is, it bonds to grease, and only goes away because it resists water. It's a pretty poor solvent. Beyond that, it's not 'green' at all if you just wash it down the drain--all that oily gunk and metal does no good for the sewage system.