10. Super Fantasy Zone
The first Fantasy Zone game not made by Sega turned out to be a massive winner. I'd been a fan of Fantasy Zone since the first on the SMS, and I remember going from store to store in 1991 on my birthday looking for the sequel (and eventually finding it at a Toys-r-us where they had one copy in stock). Super Fantasy Zone isn't my favorite in the series (that'd be the incredible Fantasy Zone II DX released in 2009) but it's still a solid entry. It has great music, and the last level is the Assembler level from Space Harrier (marking the second time a level from space harrier has appeared in the series).
9. Contra Hardcorps
While Castlevania Bloodlines was a great primer to Konami's output on the Genesis - taking a classic Nintendo-centric franchise and twisting it up for a Genesis release - I would never call it the best in the series (Rondo of blood takes the cake for me). When Konami brought Contra to the Genesis, however, they produced the greatest contra game in the series -- to this date in fact. Neither Contra 4 nor Rebirth can touch this game. Piling on an amazing amount of special effects for a Genesis game, as well as branching paths and probably the most wicked difficulty in the series, this stands as one of the best games on the entire series.
8. Toejam & Earl
The prototypical rouge-like, I can't even begin to estimate the thousands of hours I spent with my neighbor playing this game. One of the few games I'd ever describe as infinitely playable due to its randomized nature. As a single player game, it's merely good (and the reason it's not higher on this list), but it really shines in multiplayer. For years, this was probably the best 2-player game in existence. If you've never had the chance to play Toejam & Earl properly with two players, I feel bad for you.
7. Dynamite Heddy
You knew a treasure game was going to appear on this list, and while most would point to Gunstar Heroes, Alien Soldier, or perhaps even McDonalds Treasureland Adventure as their favorite Treasure game, Dynamite Heddy reigns supreme for me. The mix of humor and balls-hard gameplay makes this one of the absolute best platformers on the system. It's a damn shame this game is so overlooked when examining Treasure's classic output.
6. Tiny Toon Adventures: Buster's Hidden Treasure
This is the closest the Genesis ever got to Super Mario World, and man does this game ever come close. From the classic Konami team which brought us so many of their great Genesis hits, I suspect most people don't give this game the time of day currently because of the property which is attached to it (although anybody who hates Tiny Toons is obviously not human). This is a great game that starts off easy and gets monsterously hard near the end. I 100%'d this game as a kid, finding every exit, and I know I could never, ever do that again.
5. Beyond Oasis
I never owned this game until maybe 6 or so months ago. But I played the shit out of it on Sega Channel. Some people will point to Crusader of Centy as the Genesis answer to Zelda, and to be sure that's a great game as well (but not top 10 great IMO). Rather, I'd always champion Beyond Oasis, a game which took the Zelda gameplay and mixed it in with something that felt like Streets of Rage. Great game. I only recently beat it - I was very, very far in the game when I was playing it on Sega Channel (like the last dungeon) and then the month rolled over and they changed the game. Wonderful game, although Legend of Oasis on the Saturn is a bit better.
4. Thunder Force III
Thunder Force IV is probably better known, but I like Thunder Force III better. The difficulty curve is a bit more managable, and it was much more impressive for its time. The line scroll effect in the Venus Fire stage, along with the music, blew me away in '91. It has a killer soundtrack that could only be done on the Genesis at the time (the SNES "port" has some of the worst music I've ever heard). This is my favorite game in the entire series.
3. Pier Solar
Probably a weird entry into this list, because it's such a recent release. Pier Solar is everything that is great about the Genesis, including the homebrew scene. Masterfully coded, it fully takes advantage of the hardware, including the 32X and Sega CD. It's a wonderful example of just how great collaboration on the internet is, and how impressive homebrew can be. It's also a damn great game to top it all off, and it pulls off just about every trick ever thought possible on the Genesis (and some thought impossible).
2. Phantasy Star IV
Probably the first RPG that I really "got" despite playing PS and PSII prior, PSIV was incredibly epic. It pulled me in and never let me go. Though we ended up with a gimped version compared to the Sega CD version they originally envisioned, this has to be the best RPG of the 16-bit era IMO. And that includes Final Fantasy III, Chrono Trigger, etc.
1. Wonderboy in Monster world
This game would top my top 10 favorite Genesis titles even if we threw Sonic into the mix. I've been a wonderboy fan since the very first one on the Sega Master System. I grew up with the series, and bought every release in the series as they came out (with the obvious exception of Monster World IV). My all-time favorite game on any system, ever. It narrowly edges out Wonderboy III as my favorite in the series because it feels more like Wonderboy in Monster Land, only with Wonderboy III's mechanics. Superb game.
I could list out so many more Genesis titles without ever touching Sonic. Most people think the Genesis had Sonic and nothing else, but those people obviously didn't have Sega Channel. Consider the games I didn't even get to in my top 10 - Castle of Illusion, Shining Force II, Monster World IV, Quackshot, Landstalker, Ranger X, Rocket Knight Adventure, Shinobi III, etc.