Thanks to dsparil for compiling the stats and congratulations to people who took part in the challenge. I'm including games that were non-qualifying in red with the reasons why. I've ranked how difficult I thought the game is for anyone thinking of playing them.
Highs
* Castlevania IV (SNES) - An absolute joy to play. I rated it 9, would be a 10, but Dracula's Castle really spikes the difficulty and some issues with hit detection in the clock tower, such as falling through the cogs when jumping. Difficulty: Moderate.
* Super Mario World II: Yoshi's Island (SNES) 9/10 - Another great SNES game, beautiful graphics, top gameplay, tight controls. I know some were put off by the Baby Mario character, but other than that it's a must play for platform fans. Difficulty: Moderate.
* Kirby's Adventure (NES) 8/10 - Easily one of the best platformers on the NES. Difficulty: Moderate.
* Super Metroid (SNES) 8/10 Great game. Amazing atmosphere. I found the first 30 minutes really tough, but would be way easier if I was to play it again. Difficulty: Tough.
* The Legend of Zelda (NES) 8/10 - Total classic. Loses a few marks for some unfair dungeons and hidden areas which need to be found. Difficulty: Moderate.
* Binary Domain (360) 8/10. Great robot shooting game. It's just fun with satisfying combat, campy dialogue and upgrades. Difficulty: Moderate.
* Alien Soldier (Genesis\ Mega Drive) 8/10. Amazing graphics for the hardware. Original gameplay, but maybe frustrating. Difficulty: Tough.
* Mega Man (NES) 8/10 - Another NES classic. Scores 8/10 as I think there are better games in the series. Difficulty: Moderate\Tough.
* Her Story (Windows - GoG) 8/10 Original game with great writing and acting. It won't be for everyone, but I really like it. Difficulty: Easy\Moderate.
* Postal + DLC (Windows - Steam) 8/10. Honestly I had a blast. Graphics aren't too good, but it's more fun than I expected. Difficulty: Moderate.
* Left 4 Dead + DLC (Windows - Steam) 8/10. Really fun zombie arena shooter. Not really innovative, but plays well and it's great fun in co-op. Difficulty: Moderate.
* Limbo (Windows - Steam) 8/10. Surely a future classic? Difficulty: Moderate\Tough.
* Painkiller: Black Edition (Windows - GoG?) 7/10. Only a 7/10, but I think this is a fun enough game to recommend as a high. I've become a bit indifferent towards FPS, but this offers enough for fans of the genre. Difficulty: Moderate\Tough.
* Sonic CD (SEGA CD) 8/10. Another classic. One of the best after the original trilogy. Difficulty: Moderate.
* Shinobi III: Return of the Ninja Master (MD\Gen) 8/10. Best in the series? Difficulty: Moderate\Tough.
* The Darkness II (360) 8/10. Not as good as the first game, but still great IMO. Difficulty: Moderate.
* Alpha Protocol (360) 7/10 Borderline between a high and a mid. Overly long, but other than that well worth it. Difficulty: Moderate.
* Final Fight (Arcade) Too many credits. 8/10. Great game. Difficulty: Moderate.
* Darius Gaiden (Arcade) Too many credits. 7/10. Great music and graphics. Rated 7 due to repetition, but a must play if you like the genre. Difficulty: Moderate\Tough.
* Golden Axe: The Revenge of Death Adder (Arcade) Too many credits. 8/10. - Great, probably best in series. Fully realised with many improvements over the first game. Difficulty: Moderate.
* The House of the Dead (Arcade) Too many credits. 8/10. Arguably Sega's best arcade shooter. Just great fun with excellent horror movie atmosphere.
* Metal Slug 3, 4, 5 (Arcade) Too many credits. 8/10. Completing the ones I've missed they're all pretty much the same game. Difficulty: Tough.
* Street Fighter II: The World Warrior (Arcade) 9/10. Replay, but only completed with one character. Ryu. Masterpiece, but not a 10\10 as I think Turbo is better. Difficulty: Moderate\Tough.
* Virtua Cop 2 (Arcade) Too many credits. 8/10. Arguably best in the series. Difficulty: Moderate\Tough.
Mids
* Little Nemo: The Dream Master & Little Samson (NES) 7/10. A lot of hype over these rare titles. No doubt close to great, but for me they lack something compared to Kirby or SMB3 on the NES.
* Zelda II: The Adventure of Link (NES) 7/10. Borderline high, but just frustrated me in places. Possibly the toughest Zelda game with a lot of overpowered enemies and random battles (how we love those). Difficulty: Tough.
* Lollipop Chainsaw (360) 7/10. Another borderline high. I don't think it's as good as some reviews will have me believe. It is fun. Difficulty: Moderate.
* Teenage Mutant Ninja Turtles (NES) 6 or 7/10. Difficulty mainly lets it down, but charming and faithful to the franchise. Difficulty: Tough.
* Blaster Master (NES) 7/10. Fun Metroidvania game before that was a term used. Bit of a slog to get through, but charming enough to recommend. Difficulty: Tough.
* Super Star Wars (SNES) 7/10. Amazing soundtrack, tedious end level and lightspeeder segments. Fun enough to recommend. Difficulty: Moderate\Tough.
* Wendy: Every Witch Way (GBC) 7/10. Bright, colourful and a fun game. Just way too easy and possibly too short. Difficulty: Very Easy
* Michael Jackson's Moonwalker (MD\Gen) 7/10. Fairly fun with great midi soundtrack. Tedious in places. Difficulty: Moderate\Tough.
* Fantasy Zone II: The Tears of Opa-Opa (MS) 7/10. Solid for the platform, but not really what I would call great. Difficulty: Tough.
* Warhammer 40,000: Space Marine (360) 7/10. Decent polish TPS. Letdown by being overly long, really drags. Amazing production values across the board, can't fault it on a technical level. Difficulty: Moderate.
* Chuck Rock II: Son of Chuck (MD\Gen) 7/10. Fairly mediocre platformer, but a great soundtrack. Difficulty: Moderate\Tough.
* Shinobi (Arcade) Too many credits. 7/10. Seemed harder than the MS version. Total classic. Difficulty: Tough.
* Crypt Killer (Arcade) Too many credits. 7/10. Really fun just goes on for way too long at 1hr 30 minutes for a light gun game. Difficulty: Tough.
* Lethal Enforcers II: Gunfighters (Arcade) Too many credits. 7/10. Fun, but a bit forgettable. Difficulty: Moderate\Tough.
* Cyber Police ESWAT (Arcade) Too many credits. 7/10. I remember this being decent. Difficulty: Moderate.
* Rastan (Arcade) Too many credits. 7/10. Decent Conana the Barbarian style game, but too many annoying design choices. Difficulty: Tough.
* Thrill Kill (PS1) only completed with one character. 7/10. Something about it I liked, but difficulty to get through as it's sort of boring after a while. Difficulty: Moderate.
* Golden Axe: The Duel (Saturn) only completed with one character. 7/10. Amazing graphics for the time, but not that fun to play. Difficulty: Tough.
* Eco Fighters (Arcade) Too many credits. 7/10. Fun Capcom side scroller, but not a great game. Difficulty: Tough.
* RoboCop Versus The Terminator (MD\Gen) Cheats used. Frustrating difficulty and level design. I couldn't complete it without cheats. Difficulty: Tough.
* Gaiapolis (Arcade) Too many credits. 7/10. Forgotten infusion of RPG Lite and action adventure. Too many credits used on end boss. Difficulty: Moderate.
* Alien Storm & Alien Syndrome (Arcade) Too many credits. 7/10. Two of Sega's fun alien games well worth a go. Difficulty: Moderate\Tough.
* J.J. Squawkers (Arcade) Too many credits. 7/10. Cutesy platform fun.
Lows
* Undercover Cops (Arcade) Too many credits. 7/10. Good, but forgettable. Difficulty: Moderate.
* Dynasty Wars (Arcade) Too many credits. 7/10. Good, but forgettable Capcom game. Difficulty: Tough.
* 11th Hour (PC - GoG) 7/10. I've rated it 7, but I don't think it is worth playing now unless you absolutely love point and click horror. Difficulty: Moderate.
* One night, hotsprings (PC - Steam) 7/10. I've rated it 7, but it's very short. Difficulty: Easy.
* Operation Wolf 3 (Arcade) Too many credits. 7/10. Pretty decent, but the worst of the lightgun games I've completed this year. Difficulty: Tough.
* Kenseiden (Master System) 6/10. I had quite a bit of fun with this one. It's OK, but a lesser Shinobi style game. Difficulty: Tough.
* Power Instinct Legends (Arcade) only completed with one character. Depending on my mood either 6 or 7/10. A bit tedious, but great humour and graphics in this SFII style game. Difficulty: Tough.
* Yes, Master! (PC - Steam) 5/10. Weird Hentai game. Not really recommended. Difficulty: Easy.
Gave Up On:
* Zombeer (PC - Steam) Really difficult sniper section. I might comeback to it as I am about 70% through it.
* Half-Life (PC - Steam) This was free on Steam. I didn't find the platforming elements fun. I may give it another go, also tougher than I expected.
* Unreal Gold (PC -GoG) I will probably restart this one, just couldn't get into it.
Special Mention:
* Klonoa: Door to Phantomile (PS1) I actually completed this, but forgot to add it to the list. Oh well. Great apart from the last level. 8/10.