For modpacks, my all time favorite is Treeit, but Tekkit Lite back when it was new was my favorite back then. I love all the mods in those packs and nothing conflicts with any other mod (also Treeit in the next update is going to have my mod Quartz 'N' Steel, which I'm happy about). It's fun to play, not too easy or too challenging, and multiplayer is the best. Also booze. Lots of booze.
As for my least favorite, Attack of the B-Team. I know most of you will be pissed, since it just came out and is really popular, but I just hate it. It's overpowered, has almost no progression or reason to do anything, it's missing lots of mods (and has a few that I'd prefer if they were missing), and it's generally just crappily put together.
As for mods, I can't really say what my favorite mod is overall, they all have different uses, and I'm not going to be a dick and say my mod is the best.
I'd have to say Buildcraft and IC2 are among the best, because they're basically essential to modpacks and have so much mods built off of them.
Other mods I like are ExtrabiomesXL and Biomes O' Plenty for the new biomes they add (those mods together, along with Highlands which is like a EBXL expansion, make over 200 biomes!), GrowthCraft because of the new, not overpowered farming options (also booze.), and Special Mobs because it makes the game more challenging, and not many mods these days are strictly challenge adding.
There's also some mods I like because of their game mechanics, but kind of dislike because of the way they were made.
Thaumcraft, for example, is awesome but the models/textures are ugly and annoying (this goes for any mod with non-16x textures and not cube-shaped models, because it doesn't look Minecrafty, but it's not so bad).
I feel the same way about Tinkers Construct, because the custom tools and stuff are great, but it's a bit overpowered (3 iron for a vanilla iron pickaxe, 1 iron for a tinkers iron pickaxe; free repairs and tool renaming without an anvil; tools never technically break) and ignores vanilla game mechanics (why can this sword be made on a crafting table, but this other sword needs 3 special blocks, a casting mold, etc.?).
Also Galacticraft is cool because you can go to space, but the mobs and other features of the new planets weren't very well designed, and the bosses are stupid. However, I still generally like those mods, despite their flaws.
As for mods I really don't like, I'd have to say Dartcraft, Advanced Genetics, Mo' Creatures, and Modular Powersuits. I'd include Morph on the list, but it wasn't really meant for balanced survival use. That would be like hating on the Dirt to Diamonds mod.
Anyway, I don't like Dartcraft because it's useless, overpowered, stupidly made, and generally terrible.
As for Advanced Genetics, just no. It's probably the only mod that I absolutely hate everything about. It's way too overpowered, as well as contradicting its own usefulness, the main premise is stupid, and it completely ruins the game because why do anything anymore if you can fly, teleport, are immune to almost everything, never run out of hunger, and have infinite potions? Also the texturing is horrible, probably the worst of any mod I know of, but that's barely worth mentioning compared to how bad the rest of it is.
Anyway, ranting aside, the other two mods, Mo' Creatures and Modular Powersuits, I don't really *hate*, but I dislike them.
Mo' Creatures mainly because of the textures and models, which I know I've been focusing way too much on in this thread, but it's really annoying to play and enjoy it when everything is so ugly. It also goes against vanilla Minecraft in its ugliness, because the wolves and skeletons it adds look nothing like vanilla skeletons and wolves. I don't know what Dr. Zhark was thinking, maybe he wanted to show that he puts effort into his mod instead of copying vanilla, but it's really annoying and looks terrible.
As for Modular Powersuits, I love the mod itself, but I dislike when it's in modpacks, especially in multiplayer, simply because it removes the need for any other kind of armor. Also if you enable HD textures, it looks really ugly, this is Minecraft not some Halo ripoff made in the 90s!
So what are your favorite and least favorite mods and packs?