I mod just about everything. D:
At first it's mostly clothing/hair packs, guns/ammo packs, different weapons in general.. Customization things, really. I don't want to mess with the game's story before I even play it, but I am a fan of cute things. And guns. <3
After I beat a game, I'll take bigger mods into account, sometimes just to try them. It didn't last long, but for a couple days playing Fallout 3 again with a zombie apocalypse mod + "Real Time Settler" + something about grabbing anybody as a companion + adding cooking, New Vegas style.. made for a pretty neat experience. For a few days or so.
With Sims, I'll wait about a week or so on the base game before I add bigger things to it or do more than console commands. I still want to experience the original, but let's face it, Sims gets boring real fast without a lot of help from the mod community.
Skyrim was the exception. I pulled out the stupid "arrow to the knee" dialogue from the get-go.
And new guns/ammo/etc weapons are usually my faves. Especially when the mods actually resemble its real-life counterpart more than the ones in the game. Or just to add in personal favorites that didn't come with the game. Ya know? :3