Well, I have never used any of those markers, but from what I've seen around me, I can tell you this.
Any Spyder is a good marker to start out with. They are pretty well made, have a good service back-up, and are very upgradeable, since upgrades and add ons are readily available from most paintball suppliers.
The Core Sharc I have never heard of, much less seen in action, so I can't tell you anything about it.
The Black Dragun, from what I understand is also pretty well made, but has very few bells and whistles (meaning gadgets that'll look the part, but won't add anything to your game). What's cool about that marker is that it has an electronic trigger, meaning it's far easier to achieve high rates of fire. It'll be a lot easier to get on a level playing field with some of the high end markers with a marker like this, than it would be with a regular blow back semi.
The best piece of advice I can give you though, is see if you can try out those markers on your local field and see which you like best. After all, what works for some, may not work for some others.
It may also be worth it to have a look in the techroom section of this site. Tons of markers have been reviewed in there, including the Dragun.
Have fun playing bro!
