Sinistron/Violent Soldier
Tricky Kick
Jimmu Denshou
Altered Beast
Image Fight
Genpei Toumaden
SideArms
Victory Run
Deep Blue
Super Star Soldier
I had to reach a bit to get to ten. These aren't necessarily in any order, but Sinistron at the top seems right.
I was on the fence when it came to including Genpei Toumaden. It's "hard" because of all it puts you through with its penalty zones, horrid mechanics, and warps back to earlier levels (so basically, for all the wrong reasons). Most of the others are what I'd consider "good challenges," although a few of them certainly have their cheap spots.
I haven't played PCE Tatsujin yet, hence its omission. And Raiden probably deserves a spot, but technically, I've played only Super Raiden, so I left the chip off.
We should do lists for the CDs, too.