Kneepads are very personal. What works for one person doesn't for another. Plus, having had knee surgery, I probably need more knee protection than some people. I want to be able to crawl comfortably on chert and popcorn. And I only considered knee pads worn on the outside. So here is my experience (worth exactly what you paid for it):
I found the Expedition Essentials mentioned by the previous poster will not stay in place (both to the side and up and down). They might be okay for those will bigger legs - if you have even moderately skinny legs, these will never stay in place. The stitching is not that good and you may want to run over it with a sewing machine before using.
The Howitzers do stay in place, but they are too thin a pad for me - good for the trips where people insist you don't need kneepads (I've learned never to believe that).
The long Dirty Daves are comfortably thick, but lack side coverage, and they seem to wear quickly on the back and at the seams.
The B&C Wunderwear kneepads (middle length) require religious cleaning of the velcro after muddy trips (especially after several years), but provide good protection to the side and do stay in place. The new version has easy to replace straps. Have worn out velcro and elastic strap on my old pair, but the pad part is still good.
Got a pair of Cave Legs knee pads at the convention. They are a very nice quality and a step or two above Dirty Daves in design, but I haven't tried them yet.
My first 20 years of caving was done with Rockmasters. If they provided shin protection and if those dang elastic straps were easier to replace I might still be using them - they really do protect your knees. They sure took some getting used to at first
But in those days there was not much else that worked.