I usually give it a watch here and there during the 24hrs. My favourite race is the GT Challenge EU Spa 24. The past couple of years, 1st and 2nd place have been finishing within seconds of eachother.
Edit; it's amazing watching a NASCAR roaring through Le Mans!
I can't get myself to watch GT-only endurance races, but definitely the GT classes are the highlight of any multi-class race... The competitiveness is crazy. At the last one, I forget if it was WEC or 24h Series, there were fights for position up to the end in the GT classes. LMP not so much.