The Must Have Megadrive games!

[CaptainAmazing]

Gley Lancer
Alien Soldier
Decap Attack
Ex- Ranza
Atomic Robo Kid
Eliminate Down
Yuu Yuu
Dynamite Headdy
Devil Hunter Yohko
Monster World 4
Tatsujin

[Kaneko]

Gunstar Heroes
Contra Hard corps
Twinkle Tale
Akuma繝ｻ

[Azazel]

Elimanate Down
Golden Axe 3
Mercs
Rocket Knight Adventure
Landstalker
Story of Thor

[Recap]

Decap Attack

Magical Hat no Buttobi Turbo! Daibouken, instead, please. I'd add Gynoug, Pulseman and Ragnacenty. Some people say Fushigi no Umi no Nadia, Bahamut Lagoon and Blue Almanac are great. I don't know if it's true, thoe.

[zinger]

Adding a couple of nice games that came to my mind atm:
Battle Mania II
Splatterhouse III
Ristar
Devil Crash MD
Slap Fight MD
Golden Axe series
Valis III
Undead Line
Midnight Resistance
Chelnov
Puyo Puyo II
Herzog Zwei
Star Cruiser
Mega Turrican
Phelios
Gain Ground
World Of Illusion
Caliber Fifty
Granada

[Gaijin Punch]

I'd drop El Viento and Phatnasy Star III and add:

Snow Bros.
Rainbow Island Extra (cheapest port out there)
Gain Ground

[Bighab]

I'd add:
The Hybrid Front
Vixen 357
Dahna
King Colossus

[zinger]

Ah, Vixen 357! Such a nice intro, and such poor gameplay! Does it ever pick up? I played the first three battles and was bored to death. The intro music is one of the best VGM tunes I've ever heard, but in-game, its just terrible... Does it gets any better, or have I missunderstood the game? And what about Devil Hunter Yohko, Atomic Robo Kid, Chamelon Kid, Valis SD and Moonwalker? I always fail to see any quality in these games

[Azazel]

There alright titles but probably not must haves.

[ArnoldRimmer83]

Monster World IV (aka Wonderboy 6)
Wonderboy 5
Phantasy Star II & IV (III isn't completely terrible, but very lackluster to the other two.)
Gunstar Heroes
Ghouls n Ghosts
Forgotten Worlds
Mercs (Loss of multiplayer hurts it, but it does have a new mode not in the arcade.)
Marvel Land
Dangerous Seed
Burning Force
Phelios
Splatterhouse 2 & 3
Rolling Thunder 2 & 3
Gleylancer
Eliminate Down
Gaiares
Thunderforce series
Musha
Puyo Puyo 1 & 2
Castlevania Bloodlines
Contra Hard Corps (Decent, but wish there was less emphasis on boss fights.)
Rocketknight Adventures and Sparkster
Sonic series
Shinobi series
Mega Turrican
Spiderman vs Kingpin
Bare Knuckle/Streets of Rage series
Ristar

Rent a Hero looks neat, but with no translation available I can't play too far in it.

[Pyongyang]

Arrow Flash is excellent!

[muki]

Space Battleship Gomora / Bioship Paladin

If you like Atomic Robo Kid, this has that definitive Fujisawa/UPL art style and a great 2p mode as well.

Chelnov / Atomic Runner
Data East, what more needs to be said?