Oh, I sure can't top those replies!
1. Your personal interests
2. Your expected cash investment
3. Storage/display space
4. Spousal tolerance for yet another knife
The most expensive is, of course, one of each. There are literally thousands of Schrade patterns before you delve into the siblings Ulster and Imperial. And then of course, there are the contract SFO knives for Sears (Stamped or etched Sears, Craftsman, J. C. Higgins, Ted Williams, etc.), Hoffritz, Simmons Keen Kutter, Wards, and on and on. Let's not forget founder George Schrade's Geo. Schrade, Presto, FLylock, and many more. And there are factory customs, prototypes, samples, and mistakes, then the "last of days" knives. And even Schrade can be divided into stamps of Schrade, Schrade-Walden, Schrade Cutco. Unless you have a money tree, specializing is probably the best economically viable way to collect. I have a quite varied collection, but my focus is really on the 165OT and 15OT patterns. That does not mean I do not indulge myself in a nice folder or fixed of another pattern when one comes along, but the variations in one pattern like these would astound a person unfamiliar with them.
Codger