My main ‘overhaul mod’ is OOO – I like how it balances things out. I like the new quests, armors, weapons and all the little details it adds to the game (like the posters and pamphlets addressed to adventurers).
I fell in love with MMM for what it adds on top of OOO – mainly new creatures, diverse mobs (groups of skeletons or gobelins with different sizes, colors, etc), other adventurers that can help you… or not.
I wouldn’t mind to see some of the additional weapons and armors from Fancesco’s (or from Warcry more accurately), but since I already have more armors and weapons to look for than I can care about, i can live without it.
In short – I use OOO for the richness it adds to the game and MMM for the life it brings on top of OOO.
If you are ambitious and want all these features at the same time, look up the FCOM project – a complete integration of four major overhauls and an attempt to integrate the best of these mods.
Edit: For a different overhaul experience, with lighter system requirements, I suggest you look at T.I.E (Tamriel Immersion Experience). If I wasn’t already running MMM and OOO, I would definitely give this one a try.