I agree with already posted sentiment.
If nothing else without John McCain there is no pResident Zero.
Amnesty, cap and tax, gang of 14, Keating 5... all good reasons but how can you argue his being a horrible Presidential nominee trumps any and all of that.
I think it can be argued that without the base saying “My guy is an awesomely awesome bag of awesomeness and I won’t coalesce behind anyone else” until we were down to Romney and Huckabee as “conservative” alternatives, there would have been no nomination for McCain.