Mostly Neo Geo and SNES games. I have been getting better about buying Genesis games but there are only like 20 of them that I really want and I have 8 of them so far. NES I tend to pick up a few favorites here and there, which is only at 47 carts.
Lately, I have been going wild with the Neo Geo. Picked up Real Bout and Real Bout Special for AES the past 2 weeks. All I need is Real Bout 2 and my main SNK fighter collection is done (AOF, SS, KOF, FF, LB, WH). So far, my AES collection holds at 40 carts. I just really buy the games I know I will play, which the Neo tends to have a good amount of at around 70% of its library. I pretty much have gotten the harder to find or more expensive carts out of the way. The rest is the cheaper and more common games like, Robo Army, Last Resort, 3 Count Bout, Mutation Nation, Savage Reign, Cyberlip, Baseball Stars 2 and other games of that sort.
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When buying an AES system, try to get one that has a really good cart slot. I highly suggest on at least a S-video mod since the standard composite video of this system is that of a Genesis or worse. The Unibios also is a big plus to get blood in games like Samurai Shodown and also having English text to go with it. Check the Neo Selling Forums for some of the good honest sellers. They should be easy to get a hold of. You can find one modded out reasonably at $250-$300.