These two are my favorites. I use the Knight Force the most because it is on my primary upper, but I also like the Hi-Lux very much as well. I have not tried or handled an Athalon, but I have also handled the Leupold and March.
If at all possible, go handle them on an upper and then decide. I know that isn't always possible for some folks, but getting to a XTC match ahead of time should be possible and there you should be able to examine several kinds.