I disagree about Panzer Dragoon Saga. It's more like 17 hours, and totally worth the price if you have the money.
I agree with this. I'm not terribly experienced when it comes to role-playing games, so it's been a considerable journey. Even then, you'll definitely want to replay it, because there's a plethora of hidden items and objectives you will have undoubtedly missed the first time 'round. That and it is a great fucking game! Panzer Dragoon Saga may not be as lengthy as Final Fantasy VII, but it's well worth the time and effort.
As for rare Genesis/Mega Drive titles, Micro Machines (North American) isn't terribly hard to find. There were four copies at my local retro joint (eStarland) and they were going for less than $10-$15 a pop. However, trying to find the cart with additional controller ports is a bit trickier.
Another rare and pricey title is The Punisher. The Mega Drive version is very expensive, while the Genesis version is starting to rise. From what I've seen on YouTube, it's a good old-fashioned beat-'em-up. Such a shame, because Punisher's my favorite Marvel hero. WHY MUST YOU BE EXPENSIVE?!
Contra: Hard Corps and Castlevania: Bloodlines aren't terribly rare, but they can be expensive if you're looking for complete copies. I've owned them both and they are fun games. Sadly, they went up in a fire seven years ago, so....
Time Killers is an expensive and mildly rare Genesis. It was delayed several times, only to be released in limited quantities towards the end of Genny's life-span. It's a horrid port though.
Those are the ones I can think of off the top of my head.