I started with a 10-point system (to match my IMDb ratings) and then I briefly switched to a descriptive system (Good, Very Good, et cetera) and now I use combination of the descriptive and a 100-point scale (to match my Criticker ratings). My scores are roughly equivalent to school scores, where anything below 60 is a fail. I kind of hate my system now, because there's way too much room in that 0-60 range and my 80-89 range covers a broader spectrum than it should. But changing it would involve too much time and effort re-adjusting my Criticker scores, and I'd still have to stick to a 10-point scale on IMDb anyway, if I wanted my ratings there to mean anything.
96-100 = Masterpiece: A sublime experience, a top 100-worthy film. To me, almost every second of this movie is perfect.
90-95 = Great: A top 250-worthy film (and in fact, there are a few personal favorites in this range that made it to my top 100)
80-89 = Very Good: Anything from "Hey this was pretty good!" (recent example: Pitch Perfect) to "This was superb, just one or two small things holding it back" (Tale of the Princess Kaguya). I would say most movies I watch fall into this range.
70-79 = Good: Enjoyed it, but probably wouldn't give it a second look. Recommended mainly for fans of (director/actor/genre/subject matter)
60-69 = Fair: Some redeeming qualities for sure, but too many issues to ignore.
30-59 = Poor: Far too problematic, but maybe I enjoyed one or two things about it
0-30 = Crap: A miserable experience. This movie pissed me off.
Actually, all the polls and rankings lately have got me thinking again about the arbitrariness of assigning numbers/grades to movies. But it's such a habit, I find it difficult to let go of.