Mod: Mekanism (also partial to Ender IO, Applied Energistics 2, and Minefactory Reloaded though).
Minecraft Version: 1.7.10
Description: Your basic tech mod. Adds power generation, machines to automate things, etc. Also adds some very cool tools, like the paxel (all in one mining, digging, chopping, hoeing, fighting tool) the atomic disassembler (powered version of the paxel with various settings), jetpack, and free runners -boots that use power to absorb falling damage and allow you to walk up one block without jumping.
I spent all of about 2 weeks playing vanilla before I started adding mods. I'll probably go back to it occasionally at some point in the future, but for now, I'm having too much fun with mods.
It's irritating to have to click on a spoiler in every post just to see something that is neither a spoiler, that is something that you want a chance to avoid seeing, nor big enough to make it hard to find posts in the thread.
My favorite mod is TFC (Terrafirmacraft), which is a complete rework of minecraft, making it more, as the mod states, believable, which is the common misconception. It doesn't try to make it realistic, jut believable. I love how challenging it is, and it adds a new "vanilla" for me.
Just like Tallenn, I love Mekanism, Ender IO and Applied Energistics 2, but I also like ICBM and MineChem a great deal.
These mods with the exception of MineChem for the moment are 1.7.10.
ICBM is about blowing everything up with various missiles. Included in the package is some great technology to defend yourself. MineChem is chemistry at its finest in Minecraft. Ever wanted to know what's in that item? Find out. Running low on one material that shares the same elements with an item you have too much of? Convert it. Want to turn coal into diamonds? You can do that too! Depending on the version, poisoning food is also a possibility.