Zvevda kits are good enough. They are accurate overall but don't have the nice fit of the modern asian kits. I don't know if the Zvevda Su-27 has raised or recessed panel lines. That might be a consideration for you. They do fit better than the Monogram kits of the late 70's and early 80's. Airfix has a decent Su-27. You can find them around still. It is not too bad but once again doesn't have the fit of the asian makers. The problem with the asian manufacturers is that they tend to get the overall shape wrong.
As J-Hulk said, the Academy Flanker is a very nice kit. But at 1/48 scale the Flanker is a big aircraft.