Splatterhouse 1/2/3 for the Sega Megadrive/Genesis.
I've never seen anybody mention those games, like, ever. Even with those 'retro' youtubers and their Halloween specials, this never gets a mention (except for Cinemassacre who covered 2+3 this year). Even when the reboot came out a few years ago, people thought it was a new IP.
This. I love the Splatterhouse games so much. I discovered the series years ago and wish the three original games got more recognition. The reboot was of questionable quality in terms of plot (pretty dumb to be honest) but the gameplay is enjoyable for what it is. The real reason to buy the reboot though is to get the original trilogy for a really cheap price. It didn't sell well due to general obscurity and poor reviews, so you can find it for like $10. $10 for a decent game and three great classics? Sounds like a good deal to me!
I realize Fire Emblem has gained popularity thanks to Awakening, but Awakening honestly has a terrible plot. The older games are far more interesting and had better characters. I wish more people would try out the other games, especially since you can find two of them (Fire Emblem 7 and Sacred Stones) on the Wii U Virtual Console. In particular, I recommend Path of Radiance and Radiant Dawn for the Gamecube and Wii respectively. PoR and RD are the games where Ike is from, so anyone who likes to use him in Smash Bros. could benefit from learning what his games are like. Fire Emblem 4 probably has the best plot in the series, but it was exclusive to Japan and has never been officially translated. Path of Radiance and Radiant Dawn are right behind it in terms of interesting story, in my opinion. All this plot talk aside, I've always been a fan of tactical RPGs and Fire Emblem games are definitely solid choices in this department.
The Gargoyle's Quest games are often overlooked but are definitely something anyone who is a fan of retro platforming should check out. Gargoyle's Quest 1 is on the Game Boy and can be found on the 3DS Virtual Console. Gargoyle's Quest II was on the NES and can be found on the 3DS and Wii U Virtual Consoles. Demon's Crest was on the SNES and can be found on the Wii U Virtual Console. If anyone played Ultimate Marvel vs Capcom 3, the character Firebrand is the main character of the Gargoyle's Quest games.
As someone who supported Shantae in the Smash Bros. Character Ballot, I would be remiss to not recommend her series. I discovered them a few years back and they're pretty fun platformers. Her games have been on Nintendo consoles since the beginning on the Game Boy Color. You can find the original on the 3DS Virtual Console. Risky's Revenge was released on the DSi and is coming to the 3DS and Wii U Virtual Consoles this year. The third game, Shantae and the Pirate's Curse is on both the 3DS and Wii U Virtual Console. The next game, Half-Genie Hero, is coming out later this year or early next year.
Anyone who is or was a fan of the more horror-themed Resident Evil games should know that the original idea behind Resident Evil was to remake an NES game known as Sweet Home. While it was only released in Japan and never officially translated, fans took it upon themselves to translate it. I don't recommend piracy, but if nothing else it would be nice to see the game get more recognition. Maybe check out gameplay on YouTube? I wish they would put it on the Virtual Console, but alas.
Finally, I saw mention of Zombies Ate My Neighbors here. While I love that game, I wouldn't necessarily say it's underrated. It might not be generally known by a whole lot of people, but it is often regarded as a very good game whenever I see it brought up in discussion. That said, anyone who hasn't tried it before ought to give it a shot.