I'm having a lot of fun with the new modpack, just because it's so...fresh. No old and bland mods like IC2 or Buildcraft, and lots of cool new ones that I hadn't even heard of, like Witchery.
My least favorite is probably Hack Slash Mine. Trying to play WoW in Minecraft just seems...weird.
I greatly enjoy playing on hardcore PVP maps, which is why I love Flan's Mod and ICBM. For the same reason, I detest mods like IC2 for the quantum suits, MFFS for the ability to make inpenetrable forcefields, and Galacticraft for the ability to make a stupid spacestation that nobody else can even get to. I know it isn't technically a mod, but Factions sucks too. Without using exploits or treachery, it is literally impossible to get into a decently built base and raid it. I'm all for mods that add traps and things to help fortify your base, but at the end of the day I think that you should be able to take over any base given that you have enough people and are well equipped.