maybe not exactly a 'must have' tool but an air compressor is about the most versitile tool there is.
wish i had one....

 

air compressor is great, i love mine. ratcheting metric wrench set has been a god-send as well. also important are 6 point sockets, not 12. i was amazed at how fast those bolt heads can strip under normal loosening/tightening. especially the lugnuts.

my favorite tool of all has to be zip ties[img]i/expressions/face-icon-small-smile.gif[/img][img]i/expressions/face-icon-small-wink.gif[/img]

 

i work on atv dirt bikes all the time aside from my own fleet i do all my own work and out sorce nothing so this is some of the tools you might consider getting,aside from sockets ratchets and wrenches,
brass,lead and copper hammer,impact screw driver, cylender hone, ring compresser valve spring compressor, vacume gage, cylender leak down tester, tork wrenches, bearing seperators, cable luber, chain breaker, chain alignment tool, test light, multi meter, peak voltage reader,flywheel pullers,grease gun with needle injectorsnap ring pliers both inner and outer, shock spanner wrench,air blow gun,air dart with the scuff pads which work great for gasket removing and cleaning,also shop press and parts washer are very usefull.just a list off the top of my head.