I used to use the Beaver ones, but the Velcro always ripped off quite quickly.
I?ve since used the Warmbac Warmtex pull-up kneepads. They are great for sporting caving where crawling is intermittent and generally smooth, but for digging in sharp/gravelly rough conditions, the straps rip off completely after about 20 trips, long before the Warmtex has worn through. Therefore, before first use it?s always good to liberally coat the outside of the kneepad (where the Warmtex meets the fabric surround and straps, front and back) with neoprene glue. Makes them last three times longer for only ?5 worth of neoprene glue.
This doesn?t stop the classic issue of them slipping down while crawling though, although I have always used stopping to pull my kneepads up as a sneaky excuse to stop for a quick breather!
Lately I have been moving more towards wearing a cheap and more comfortably fitted sports kneepad (sleeve-type) under my oversuit, which doesn?t slip down in the same way over-oversuit ones do as they are better fitted. The downside is that you very quickly trash your oversuit legs without any protection. Therefore, when I get my next oversuit, I am going to look at stitching reinforcement across the knees (a few layers of material sacrificed from my old suit should do the trick), and then to just wear the more comfortable sports-type kneepads under the oversuit.
In summary JefeBo55, go with whatever gives you a comfortable fit particularly if your size makes finding a good fit difficult, but consider doing some reinforcement work to the kneepads when they are new out of the packet (e.g. coating with neoprene glue) to increase their longevity OR consider wearing kneepads under the oversuit, and reinforcing the oversuit knees to compensate.