I'm the opposite. I love rounds where the other team is either decimating mine or putting up a good fight because it means I don't have to hold back. I find rounds where my team is dominating to be boring.
I'd say that close matches are the most fun.....by a mile. But, if it is a very lopsided match, I agree with KLA that it is more fun to be on the losing side. You can just go balls to the wall and see what happens. It doesn't matter if you die, cuz you were gonna lose anyway.
I suppose that logic should apply just the same in a match where my team is way ahead. The thing is though, in a win, I want to feel like I contributed. So, I don't feel like I can play as freely as I can when my team is way down.