A lot of people who grew up in the late 80's and early 90's undoubtedly remember the Genesis, or Mega Drive as it was known everywhere else in the world except for the US, and it's one of the 16bit consoles with the longest lifespan, together with the SNES. The reason I think is that it had a lot of great games for it, mostly pretty decent arcade conversions like Afterburner, Altered Beast and Golden Axe. And of course, we have the Sonic the Hedgehog games which in many ways I prefer to Mario games, but I like both equally.
Due to the popularity of the Mega Drive you can easily find it on eBay and other auction sites, and probably the odd second hand store and yard sale will feature one. It's very affordable and there are several versions of it. The model 1 Mega Drive is the biggest one and features a stereo sound headphone output which you can also plug to your TV to get proper stereo sound for your games. The Mega Drive II is a bit smaller, and doesn't feature stereo sound output, and the Mega Drive 3 is even smaller and also has limited compability with several addons for the console, such as the Mega CD and the 32X. There was also a version released by AtGames recently, which is basically just an emulator with a cart reader and gamepad ports, and features very lackluster sound output, also in mono. This one is not recommended if you're a fan of the console. In my opinion you should get the model 1 if you can.
What I like about the games themselves is that they are very affordable and easy to find complete in the original boxes. This is largely because Sega used plastic boxes for their games, kinda like the covers for VHS tapes. Nintendo on the other hand used cheap cardboard boxes that most people just threw away so if you want to find a complete SNES game it's probably going to cost you a lot more. Mega Drive games in general tend to be much cheaper than SNES or NES games so collecting for the console is very affordable, excluding the really rare games like Duke Nukem 3D and MUSHA.
There's a ton of different accessories for the console, and a lot of controller variations. There exists several turbo controllers, six button controllers and so on, even adapters that allow up to 4 players playing simultaneously, a must-have for party games like Bomberman. As previously mentioned you can also find the Mega CD addon which allows you to play optimized CD-ROM games and also regular music CD's, my favourite Mega CD games being Android Assault, Silpheed and of course Sonic CD. The 32X addon has a lot of games with advanced graphics such as Doom and Star Wars Arcade, but in general isn't worth having unless you're an avid collector.
While the Mega Drive might not have the legendary library of Nintendo, it does feature a ton of great games and it's a great console that I highly recommend to retro gamers as it's very affordable to collect for and finding complete boxed games is very easy.