Old Kingdom was my favorite at that level. The xp there was amazing. With rest each pull gave ~10k XP. None of the bosses will give you any problem after the first time you learn them. Also it is the first instance where the blues disenchant into dream shards instead of small dream shards.

I spent most of my 73-75 in old kingdom and it went real quick. Only left to clear each other instance once or twice for quest/gear/achievements. At 76 I started clearing Halls of Lightning and all the other 80 dungeons a couple times. You will need 2 characters at least 77+ with flying to summon them there with the stone if you are under 77 but HoL is worth it for the xp and gear.