I flew with them LHR-SVO-DEL a few years ago. They were OK: nothing special, nothing odious.

I was under the impression you didn't need a transit visa for SVO, though, unless you planned on going landside. Most places don't ask for this. I certainly didn't need one.

If you are in transit for some time, though, and you're offered a free meal at the transit restaurant, do watch out for the food quality there. I had a few bits of some still-frozen-in-the-middle fish that didn't do my already weakened stomach any favours.

Compared to the airlines you mentioned, though, I'd say SU would have a quality lower than all of them (on international routes, at least).