Rigid MTBs:
Ti Kona. Not hugely different to the steel equivalent, but noticeably more comfortable after an all-day ride. And corners like it's on rails.
A housemate's steel Diamond Back. It didn't look like anything special, but had a very nice, light tubeset and was amazing to ride. Add your own "steel" cliches. It was a real eye-opener.
Road:
My old Cottingham TT bike. Again, the tubes had some multiple butting that gave it an amazing feel. But the riding position was extreme and I guess I got too old and needed something more relaxed.
29er:
My Fisher Rig. It rolls over everything and is very comfortable... but the wheels are very heavy and it handles like a lazy pig compared to a 26er.
So, from one extreme to the other. All of them have their place.