I enjoy mono-decks because I don't have to worry about quanta balance, that seems to be one of the things that takes up the most time in the fine-tuning process. Some duo-decks are fun to play, but I always feel like I'm doing something wrong quanta-wise.
Mono fire was the first type of deck I made, and is still a favourite for PVP, and probably my favourite. Monodark is my arena grinder of choice, and mono-aether is used for some NPC battles (half-bloods and some oracled false gods), as much as I hate having to play against it.