I use a 5-star game rating scale. Adjusted to BGG's 10-point scale, it comes out as this:
.10!: Outstanding game. These are my all-time personal favorites.
_8_: Very good game. These are games I play frequently (or would if I could). Generally review-worthy.
_6_: Above-average game. These games are usually worth owning, and contain some great qualities, but aren't usually my first choice.
_4_: Below-average game. Eh, not so good. I'd usually rather try something new than play this again.
_2_: Very poor game. The prospect of playing this again is almost laughable.
Top 10: These are my ten favorite games in (rough) order. I tend to revise my rankings after every few plays, but I don't always update this list.