Spyderco makes a lot of excellent knives that sell for under $100, but I'd say there is no single "best" Spyderco for under $100. You have to consider where you'll be carrying, what you'll be cutting, and who's potentially going to be around to see you use the knife. The SS Police and the Military are great knives, but don't forget that when they're out and opened you're standing there with a 9" knife in your hand. That's fine if you're out in the woods, out on a fishing boat, on a construction site... But if you're in a more public area you are likely to make people decidedly uncomfortable. Also, if you're mostly opening mail/packages, cutting string, a 4" blade is way more than you need and can actually be quite awkward. Bigger knives are also less discreet, and in many places illegal.
I'd be willing to bet you don't need a larger blade than 3", and maybe not even that much. The paramilitary is fantastic, I'm never without mine in the woods. But it's large scales and deep blade make it look like a much bigger knife. I'm having a fling with the Dodo right now, it is absolutely the best, but a lot of people thinks its fugly. Everyone raves about the Calypso Jr., I don't have one but it looks like it'd be a great EDC. The jigged bone Kiwi is really nice, good blade shape for everyday utility type stuff, and very people friendly. SS Endura for the "classic" Spyderco look. Navigator and Cricket are smaller still, but again excellent cutters, and you could buy one of each for well under $100. The Herbst is a beautiful knife, and while not in production can be had on e-bay for around $50. If you go straight to the $100 limit, you may very well fly right over the best knife for you.
Anyway, this has gotten away from me. Again, I'd first think about what you need and where you'll be using it. Good luck, and let us know what you decide on.
