hmmm, what works for me is:
2 boxes loaded to the gills to fix just about anything
2 carts to get the tools to the cars
and done enough vehicles from each manufacturer to know how it works to diagnose and fix faster
but i guess you are asking for tools.
well when i started out and even sometimes still i will go scope out google for what i'm about to do. most of the time you can find some guy on a forum that has taken step by step pics on how to do something. use your discretion though, alot of these shade tree guys miss important steps or are too lazy to type them out.
knowing how to tighten different size nuts and bolts by feel is also a major thing in not breaking bolts or under torquing them.
speaking of a great time saver is having several ratchets, stubby, short, long and x-long as well as sockets in each size.
good air ratchets are a must, a long 1/2" breaker bar, a snap on air hammer, a 16oz ball peen, a 3lb sledge, a snap on brake spoon B3404B (my fave multi purpose tool of all time), Snap on brake bleeder B1467A also great for removing the 5/16 bolt in diffs, swivel sockets, gearwrench gearboxes (0 degree
http://www.tooltopia.com/gearwrench-85988.aspx), a good beench, vise, welder
i am thinking when i get a hammerhead itll be on the list.
and most of important a local parts supplier with a knowledgeable rep, & a fast driver. my garage is only a 3 bay but me and my other 3 guys do almost double what all other garages and dealerships are putting out.