I find myself humming a few of the tunes absent-mindlessly from time to time. I really like the music in the main series. I'd have to say Gen III easily has the best music, because of the overload in trumpets, which I think was the best thing they could ever do. Second favourite would have to be Gen V though, they have good music as well. I don't think any generation in particular has horrible music overall personally.
My favourite soundtracks would probably be the Kanto Trainer theme in HG/SS or the Gym Leader music in R/S/E,followed by Gary's theme in FR/LG, Ho-oh's theme and then probably the Johto trainer one as well. I also enjoy B/W's Elite Four theme. I don't think I have a least favourite personally, although I have to say the wild Pokemon theme from D/P/Pt, was certainly interesting. :x