On PC, Defender of the Crown was one of my first major time sinks. Played it over and over. SDI as well.
On Apple ][, too many to count.. I really wouldn't know where to begin.. Actually - probably Mr Robot! I spent so long playing that game and making maps. I'd make themed maps and challenges.. I had so many disks labelled "Mr Robot Maps #17". Well.. I had one labelled 17.. I'm certain the others were labelled appropriately. ><
Other games I poured dozens if not hundreds of hours into..
all the Scott Adams Adventures
I could go on forever.
EDIT: Actually, I remember now, the first PC game I played for a long time was Snipes
! Oh how I loved that. Alas, I haven't been able to find the version I had for quite some years (an 80x25, non-hacked version). All there is floating around the web is cracked 80x25 versions which play differntly, or a 40x25 version. How I miss my Snipes..