Funny I just realised this has happened to me a lot lately. It seems I fall victim to fans saying that the sequel to a given game was disappointing and end up never buying it.

Bioshock series (never played infinite)

Uncharted series (never played first)

Gears of War series (never played first)

God of War series (never played the second)

Mass effect series (never played second or third)

Halo series (never played ODST or 4)

Onimusha series (never played 3 or 4)

Many more...

DMC, Never played 1 & 2
Metroid, Didn't play some of the handheld ones
Halo, Halo ODST and 4
Dynasty Warriors, 1-6

I still haven't played the DS Zeldas.

Zelda, but haven't played the GBA or DS titles.
Metroid, I haven't played any outside of Fusion
Castlevania, only played IV, Circle of the Moon, & Dawn of Sorrow.
Halo, only played 1 & 2

There's definitely more but that's all I can think of off the top of my head.

I've never played CoD 4 or WaW, but those games are dead anyways.

GeoW: never played any, but I've always wanted to. It's the only reason I ever wanted a 360. Actually been thinking about getting one, but...
1. Still $300!
2. Still a backlog of like 15-20 games

Ratchet & Clank: Haven't played anything since A Crack in Time and I think I'll keep it that way until the series stops being a series of filler spin-offs.

Metal Gear Solid: I've only ever finished 4, and played a little bit of 1.

Elder Scrolls: I love both Oblivion and Skyrim; I've attempted Morrowind but it's too dated for me, which means that Arena and Daggerfall would be unbearable.

CoD4 was pretty great back in the day, but even Modern Warfare 2 is a vastly superior game (just a far less innovative one).

World at War is pretty much entirely a waste of space, however.

I've been tempted to buy them directly from NIntendo on Amazon but they're still ~$45. I might wait for them to put it on the Wii U eshop.

Bioshock - I've played all three games. First one I torrented (Kill me, sorry), second and infinite I got from PS Plus. Never finished any of them. I remember in the first bioshock I got stuck in the section where you have to open the safe...It was a game breaking bug where you had to dl an update, but you would lose your save. So I just gave up.

Batman Arkham - Played Asylum, and I'm currently playing City (Just started). Hopefully I can finish.

Uncharted - Played 3, haven't touched 1/2 (Blasphemy!) Really hope for a PS4 edition of Uncharted with all three games. Would def buy and play all three in order.