Iron Maiden and Viper are NOT great playing games. They are rare and only valuable for that very reason.
Regarding Quicksilver and Nine Ball, I've always preferred Nine Ball. Now, even more because it's quite a bit cheaper than QS. I may feel different if I grew up playing Sterns but from my perspective, Nine Ball is the best playing Stern (not most valuable) in much the same way that EBD is the best playing Bally.