Well, I don't remember what my 100th game was for the Genesis/Megadrive or any other console for that matter. I did buy my 1000th game for the PlayStation a few days ago, the longbox version of NBA ShootOut. I'm just slowly finishing off a PlayStation longbox collection and if it hadn't just happened, I doubt acquiring NBA ShootOut will ever be a memorable event in my life.
As far as people loading up on cheap sports games to start off their collections. I don't know, I think most people know exactly what kind of games they want to play and start with those. They might finish off a collection by adding a bunch of sports games, because they know it's not likely they'll be hard to find or expensive. The only two Genesis sports games I remember buying are the Mutant League Football and Hockey games. Exaggerated arcady sports games are all right in my book!
I bought a Sega Genesis because I became interested in collecting console shooter games. Back then I didn't even know they were called shmups, I just thought they were so unusual and unique that there wouldn't be that many to collect. Having only owned Nintendo consoles previously, you could understand why I might think that way. So the first 50 Genesis games I bought were Japanese megadrive imports. While Googling to get a list of shooters available on the Genesis, I stumbled upon Digital Press. Here I found a discussion of the top 50 Genesis games. They seemed to know what they were talking about, so I thought - why not? 304 Genesis games later, I realize that the ease at finding complete Genesis games becomes an almost irresistible temptation. I guess it's not much different from buying PlayStation games in that respect. I'd still like to have at least one Brazilian megadrive game, maybe Duke Nukem 3d. I'd still like to have at least one Korean megadrive game, maybe Space Turtle Battleship? And I know I won't quit until I have a copy of Micheal Jackson's Moonwalker.
My most memorable Genesis experience was when I told my wife I was going to work on a National Holiday for the triple time pay to buy a Genesis game. She thought I was absolutely utterly INSANE! But that day just flew by and I rushed right home, jumped on Ebay and paid $180 for the Japanese version of Alien Soldier. A year or two later Sega and Sony would put Alien Soldier on the PS2 for around $25. I bought that one as well, but made damn sure not to mention it to my wife!
Congrats on reaching 100 games! If you fancy fantasy pinball games, be sure to pick up Dino Land. I've played a lot of them and it's one of my favorites.