91 was when NASCAR really started to hit the big time- Maxx was suddenly not the only company to produce cards. It's also the year that Racing Champions really started producing a lot of cars. A good year overall!
K&M, Pro Set and Traks all made their debuts, and ARCA got it's first card set. While ARCA is NOT a part of NASCAR, I'm including them anyway.
The 1991 Maxx set is in my opinion the greatest NASCAR card set ever. I have every card available, including promotional issues, as far as I know.
1991 Pro Set was the first set to ever include an autograph in the packs. I was lucky enough to pull one, surprisingly in silver ink. (Beckett lists them as black)