CCWS2007 is a good one. The problem with 1991, beyond its illegitimacy, is that it's just a pretty miserable mod. I doubt it would make a good candidate for a league. You can't go wrong with Champcar though.
You might consider the Ferrari 312B for a nice historic F1 season (only the one car though, of course, so a spec season). Naturally, 1979 and 1988 as well would be fine choices. It really is a shame though, because 1991 was a good year. I hope we see a proper 1991 mod soon. There was speculation that the MMG 1991 mod was released just to spite another team also working on that season (I couldn't comment on that per se, but I can't think of any good reasons why they would release it the way they did, so who knows-could be anything), but hopefully that team will release a quality mod.
There is also a 1985 beta with seven cars available if you look in the WIP section at RFC. I haven't tried it lately, so I don't know how well it works, but 1985 was a great season too.