Hard to say. I don't hate fighting a class... rather I like fighting the person behind the class, as they can make a seemingly weak class very hard to fight against.
But as a rogue I like fighting mages, warriors, other rogues, Feral druids, and prot paladins.
On a healer I like fighting against other healers the old fashioned way: A battle of endurance, who can keep their side alive the longest, because honestly, after spamming your fast heal for a good 2-3 minutes without a break, any healer will go OOM.
If I ever get to remake my Warlock, I take great pleasure in fighting Rogues, mages, and every kind of hunter except BM hunters. I also like fighting other Warlocks, obviously. No other good way to judge how good you have gotten than going toe to toe with your own class, or fighting the class that hard counters you.