Some good NES games are Burger Time, Ducktales, Mighty Bomb Jack, Elevator Action, Bionic Commando, Kung Fu Heroes, Marble Madness, Dragon Power, Chip N' Dale Rescue Rangers, Ice Climbers, Ikari Warriors, Anticipation, Kid Icarus, Ninja Gaiden, NARC, Bad Dudes, Battletoads, Mike Tyson's Punch Out! (not the one with Vanilla Ice, err... I mean Mr. Dream), and then there are the obvious ones like Mega Man, Castlevania, Super Mario Bros., Contra, Double Dragon, etc...
There's just so many to list, I know I'll be kicking myself in about an hour for leaving something out, yeah like Paperboy
For Genesis I'd recommend Quackshot (and just about any other Disney game), Mutant League Football (NOT HOCKEY!!), Kid Chameleon, DecapAttack, Shining Force 1&2, Greendog, Boogerman, Battletoads/Double Dragon, Comix Zone, Mortal Kombat (Mortal Kombat for Sega Genesis is a great game, but the best of all time has to be Donkey Kong... Donkey Kong SUCKS!!... OH YEAH WELL YOU SUCK!!... or something like that haha), Toxic Crusaders, Vectorman, Michael Jackson Moonwalker, Global Gladiators, Streets Of Rage, and of course Sonic...
The thing with Genesis is, there were a lot of great games that never really got recognition so you might be able to find a few gems for cheap...
Just stay away from Sega CD and 32X if at all possible...
Oh and I'm not sure if it were released for Genesis, but I have Home Improvement for SNES, and yeah, don't get that, under any circumstances...

-Goatman