win1251 or UTF-8 (depends on what charset the site is already using)
Most common for Russian sites is win1251 (major hosts use this as their default encoding).
UTF-8 is like in transitional period - it has advandages but along with that is not fully trusted.
My advice would be: if u gonna host ur site on a russian host - use win1251, else - use UTF-8.
Looks like utf-8 would be the best for me since I also have to deal with Japanese.