What type of cable should I run to future-proof an outlet for an air conditioner?
If you truly want to future proof, install conduit instead of cable.  This will allow you to pull whatever wire you need, once you've decided what you want. 

3/4" conduit should work fine, and would give you the option to install up to four 8 AWG condcutors (Schedule 80 PVC). If you think you'd ever upgrade to a whole house unit, this might be a good option. If you only ever plan to have a single mini-split unit, you'll probably be alright installing 1/2" conduit. Though upgrading to 3/4", would make pulling easier.