What cables and adapter combinations do you keep on hand?

what is the purpose of this question?  do you have a repair service, or maintain a site?

We do break fix / MSP for small businesses

Your own field experience should give you a good idea of what kind of leads, cables, and adapters you should keep to hand. The items you suggested are all useful, especially DP and HDMI kit, as these are becoming increasingly popular. It will mean tying up more money than you'd like to ensure that you have all the various flavours of adapter for each interface, but this is still cheaper than wasted visits and subsequent revisits! Small businesses are notorious for wringing the last ounce of service from their hardware, and if this means having legacy and current equipment freely mixed then not to worry, the repair guy will make it all work...
For small business Cat6 seems overkill; I've never done a Cat6 installation for a small business customer because there hasn't been a need to do so. I have some Cat6 plugs and sockets just in case, but I've never had to use them. Cat6 cable is less forgiving than Cat5e and less flexible, so getting it into cable-hostile environments is even more of a pain. Also, no small business I've worked for needs 10Gb LAN connectivity, and certainly isn't going to pay the premium prices demanded for such equipment.

your business should show you the ones you used most - keep a list of the orderings, and you have what you need

Thanks Philip!  

Nice to see someone had a list.  I didn’t think it was asking too much!

How do you like monoprice?

Thank you!

We're in Canada so source through distribution as the shipping and US dollar exchange rate kill any real discount we'd get with them.

Our US clients use them a lot.