The races I've rated well have generally been the ones with good midfield battles. To me, a good race is one with cars actually battling for position on the track, not a processional decided by pit stops. More off-track excursions also rate a little higher, as it tends to shake up the racing order, and provided the cars aren't too damaged to continue, means even more battling for track position. Having a winner other than the predictable one is a neat little bonus worth a point or two, in my opinion.
The fact that despite all of that, I still love a Monaco race sounds a bit odd, I know, but I just can't imagine F1 without Monaco - it's the place where the glitz, the glamour, the money and the history all come together for an extravagant display that you just don't see anywhere else.