Depends...
Mineral Spirits works well for exterior surfaces and general cleaning. However, Mineral Spirits leaves a residue film of "oil" and will redeposit contamination, so use lots of clean rags.
For spark plug's, gasket mating surfaces, anything that needs to be "squeaky" clean, I use MEK. MEK is very aggressive and will eat your paint. So be careful.
For general cleaning, I have found Simple Green Extreme, Aircraft, to be an excellent general purpose cleaner. It suspends oils and contamination very well making it easier to wipe clean.
I use to use Stoddard, and still do in my solvent tank for heavy parts cleaning, screens, etc...but it is really tough on the skin if you get any exposure, so I use it only for tough jobs and dress accordingly (gloves, long sleeves, eye protection, etc...) Amazingly enough, I have found the Simple Green to cut almost as well.