My all time favs are;
Turrican 1 + 2 - without a doubt.....
Lotus espirt challenge, which i can nearly finish (don't laugh, can't believe i could finish the original on the amiga in one sitting)
Speedball 2 (i think it is 2, get both all the same - yes it is addictive, and a great waiting for the bus / plane / train game)
Castaway, like most of the emus, has fantastic 'try it' value...
Get as many disk images as you can and just go through them and try them. You'll find more than a few gems in there... In fact that is about my favorite game to play on Castaway. I started writing down all the kewl games that worked and ended up giving up on listing them. Then I started putting the disk images aside for the ones i liked, but that was just cramping my style and limiting the amount of untried stuff on the SMC. I only keep the ones mentioned above on the SMC now..
I used to really like Hunter (the activision one?) for amiga, but all the disk images i can find for it on the ST don't run in castaway, so, has anyone found a working version of that?