My priority at first was Dreamcast collecting, to own every game I know I will enjoy or at least get a laugh out of/ so bad they're good/ just interesting in some way. I've now gotten to the point where there is only a small handful of Dreamcast games I even want, excluding any upcoming indie games of course which I pre-order when I can.
Because there isn't much left on Dreamcast I'm after now I'm focusing on Mega Drive and Sega Saturn. I also own small collections of Mega CD, Master System Game Gear, Pico and 32X games. My focus is mostly the Japanese versions as they often have thinner cases and better presentation, though I normally go with which ever version I like best, or the cheapest version..PAL games can be so expensive now.
I also own some of my favorite games in multiple regions for the different revisions and presentation. I own three Sonic 3's, two Dynamite Headdy's, three Samba De Amigos, three Space Channel 5's, and various others.
My collection is on RF generation, though it needs some updating.Also: barely any cart/disc only. Complete for GTFO. In fact on Dreamcast I've had an issue of being a bit obsessed with making sure I get the spine cards :S