Title says most of it but what are some sets that you can always go back and watch? The sort of stuff that you either find really entertaining or evangelizes the game or even the scene itself in general.
For me, Neo vs Clockwork in MvC2 represents this near perfect capsulation of the FGC and what can be so electrifying about it. If you haven't watch it either at all or in awhile, I can't recommend it enough.