On the Amiga you can't go wrong with stuff by Sensible or Team17 (except Overdrive). Psygnosis aren't bad either (except Last Action Hero, for example).
These days it's got to be Konami (Pro Evo Soccer 3 anyone? Can't wait for 4), and I like Square on the strength of Final Fantasy (especially 7 and 8).
Couldn't really put them in a particular order though, sorry!