For me the best steel to sharpen is either nice thin carbon steels like in old slippies (especially Imperial's stuff and Case CV) and also the japanese white and blue carbon steels. The slippies i love because it takes so little time and effort to get an AMAZING edge that I like doing it often. Japanese carbon steels are hard and slow to grind but seem to refine very quickly, and they too take a wicked edge. Most carbon steels are like that but those are the two that stand out for me, stainless is a huge PITA to sharpen up IMO. I sharpen up my dad's Stryker that he was issued about 8 years ago whenever i go home and lemme tell ya that ATS-34 or 154 seems to take so much more time to grind down than what i usually use. If i get a stainless knife it's 12c27, 440A/B, or the higher end chinese steels like that rough rider uses.