I tend to avoid: Anything that requires activation on Steam and any standalone DRM that requires the game to be activated using the internet.
Basically, I'll only buy DRM-free games and games with serial keys that activate offline (so they're not dependent on a server, the key is checked locally and would work on a computer with no internet access).
Mostly I buy PC games on GOG.com because I know they'll be DRM-free but I shop here for other things but take extra care to make sure what I'm buying is DRM-free or if a serial/CD key is used that it's not an online check.