Mahler is one of those composers for whom interpretation changes meaning so radically that it becomes kind of meaningless to talk about 'quality' in the absolute and quantitative way that some people often do. (Actually, talking about quality quantitatively is always inappropriate, but there degrees, y'know?).

The only Solti Mahler I'm familiar with is the CSO box on Decca, which I really just can't stand. The 8th is certainly the pick of that bunch, but in some ways I suspect that this is because the sheer hugeness of it makes it somewhat 'conductor-proof'. The more subtle symphonies (which, my mind, are 3, 7 and 9) are really quite messy and inarticulate, I think. 5 and 6, as well, although the musical argument holds up somewhat better.

My own favourite complete cycle, though, is the Gary Bertini, which I understand is similarly controversial, so I can't really talk...