I prefer my engine compartment cleaning wand that goes on the air compressor and I sue a product like TSP and water with it. If doing by hand then I would suggest something like simple green and lots of sponges. You would have to hose it down afterwards. One good tip I can offer is that even though alot of folks wrap their electrical parts in plastic and tape them up, it has been my experience that aluminum foil is much easier to work with and shape around the parts and protects just as well.
If you got chrome parts, window cleaner products works well. To loose up old dirt and oil on your engine block etc; use brake cleaner spray. That is strong stuff that will loose up any dirt in seconds. Just wipe off with paper.