I rewatch a lot. Many of the shows on my completed list have been rewatched once, still others twice, and a few even more. Those that have not been rewatched will be, when I get around to them.
My ratings are meant as a recommendations list for my own use. >8 is a solid recommendation, >6 is recommended for someone who likes anime, >4 is everything I consider worthwhile.
10 - Masterpiece.
9 - Awesome. Consistently great.
8 - Awesome but flawed. Great but flawed. Good with some great bits.
7 - Consistently good.
6 - Average. Good but flawed. Good with a stretch of boring episodes.
5 - Below-average, decent. Some good moments.
4 - Bad. Boring. Not nearly enough good moments.
3 - Very bad, but I can see someone liking this because it has decent ecchi or robots or whatever. Consistently boring with no good moments.
2 - I can't find any reason to like this.
1 - not used
I only rate sequels if their quality massively diverges from that of the original, and they are long enough - no point rating a short cash-in OVA, it's a no-brainer watch even if it sucks. Another reason for a show to lack a rating is if I last watched it a long time ago.