Absolute riff monsters, with the tightest rhythm section of all time.
Forget their 60s psychedelic stuff, the 80s 'pop' hits, and the 'Quo by numbers' from the 90s onward. For a period during the 70s they were at the top of their game and no one else even came close.
Check this out from the 1971 album 'Dog of Two Head' (particularly the breakdown and solo from 2.45 onward): http://www.youtube.com/watch?v=kA5WfrU5hek
Or this 17 minute live version of '4500 times' from Milton Keynes Bowl (recorded in 1984, though originally released in 1973): http://www.youtube.com/watch?v=zx-9b13wsXQ
Watch from about 8m15s to 12m30s - makes bands like Metallica and Slayer look like schoolkids. Absolutely killed it and it doesn't even look like they're trying.
Or, of course, 'Down Down' from 1975's 'On the Level' album: http://www.youtube.com/watch?v=muEXpv_RnUI
I don't even care that they sold the song to adverts (and even re-recorded the lyrics and video for an Australian commercial). They had a party lifestyle to pay for, and drugs don't come cheap. When the guitars kick in after the intro it's always a rush!
There's loads more I could have posted, but these tracks alone prove my point. They're written off as a pub band for dads these days, instead of being remembered for what incredible musicians they are. They're an absolute institution, and still selling out major arenas today - i'm seeing them at the O2 next week. Not bad for a band that have been around for 50 years and have been seen as a 'joke' for much of it.
Forget what you think you know about Quo and actually *listen* to them. It really is time they were re-evaluated and given the respect they deserve.