Dracula X is turbo duo, so I assume you mean 16, cd and duo.
These are the titles I think deserve to be considered the best that there is to offer (though many would disagree with some) My list is essentially u.s. based, mainly because my collection is overwhelmingly u.s. I have most hucard and cd games, about half of the duo ones (I have 145 total, about 20 imports).
1. Bonks Revenge
2. Air Zonk
3. Final Lap Twin (racing and rpg mode are superb)
4. Veigues Tactical Gladiator (very unique game)
5. Soldier Blade (most like blazing lazers and this is viewed as a sort of weaker version, I view it the other way around and think this is far superior)
6. Neutopia (take zelda and improve it)
7. Buster Bros
8. Bonk 3 (hucard or CD, CD is better, but not really worth the cost difference in my mind)
9. Cosmic Fantasy 2
10. Alien Crush
11. Cyber Core
12. Mystical Formula (import, definate buy if you liked gunsmoke for nes. Essentially the same game just updated and futuristic)
13. Pac-Land
Also, you need to buy keith courage in alpha zones. It's not a great game, but it's not bad either. There are two different types of stages (you change halfway through every level). One would make a great game, the other is average at best. But you still need to buy it. Not having keith courage is like buying an NES without buying any mario games.
Also, there is a sizeable enough difference between alien crush and devil's crush to make you hate one and love the other. Most like both, devil's crush being considered one of, if the not the best pinball games ever. I hate it though but love alien crush.
Dracula X is good, I don't have or love castlevania. Excluding it's history I don't have an opinion one way or the other on the series as a whole. Dracula X is probably the best one I've played in the series, but I don't think it belongs among the systems best, or at least my personal favorites from the system.