This is subjective really to what kind of system you want to build. I have multiple HTPC's (4) and I would say a must for them is a Logitech K400R. It is a great little Keyboard/trackpad combo that is small enough but still has function keys. This basically gives you full PC navigation and typing capability which is important to me since my HTPC's are basically just giant screened work stations and I'll use them for anything computer related not just entertainment purposes. For controllers I'm not as picky as some and I have a plethora of controllers I use XBox 360 controllers for 3 of my 4 HTPC's but do have a pair of Logitech F710's that are also pretty good. As long as its wireless for my HTPC's that's really the biggest requirement.
On one of my laptops is where I will break out my wired controllers Logitech F310 these are always super cheap and definitely worth it for the price tag.
For full Windows tablet devices (I don't get any other tablets I feel they are so much more limited in use to a real PC in tablet form) I like to get Ipega Bluetooth controllers they are priced well for what you get and mine work very well (D-Pad isn't the greatest) I have the telescoping model that I use to make one of my tablets into a LB handheld and the other 360 clone model with the phone holder in the middle that I use with the tablet that I have a mount for on my dash this acts as my cars media hub with full access to my Plex server (if I bring my jetpack) and all my games via the 200GB microSD I have installed.