My vote would be for a 710HS used. Those can be had for around $60-70, and are very high quality. If you don't want a knife that big, the Grips are great, as well, but the Ritter Grip with S30V is better, although more than a new 710, sometimes. I actually carry a 730CFHS, and I feel no need to get another Axis lock knife.
As far as popularity goes, I think the 94X series is probably tops. Tough knives with a gentlemanly look. I rarely see any used ones for sale, so that must say something.
DD