Could/should/have
Mount & Blade (Warband)
Suikoden 2
Red Alert
Metal Gear Solid
Red Orchestra
Sim City
Breath of Fire 3
Syberia
Longest Journey
Company of Heroes
S.T.A.L.K.E.R.
Train Simulator
Oddworld
Broken Sword
Men of War
Monkey Island
Worms
Constructor
Silent Hill
Resident Evil
and a bunch of others

My most recent was Zone of the enders HD collection, played both games back to back. Now they're is some overrated games for you.

Normally it would be the Resident Evil Trilogy, Dino Crisis 2 or Metal Gear Solid.

Football manager football manager...

It's not about the game, but the circumstances. I remember playing whole day long via IPX or LAN with my friends "from the block". :P

I couldn't play this long in singleplayer or online with strangers. I could play 8 hours straight, but probably not the one games only.

Instead of just asking questions, why don't you share your answers with us?

What game could you play for 8 hours straight?
None.. Not anymore..

What game should you play for 8 hours straight?
None..

What game have you play for 8 hours straight?
Civilization II, Alpha Centauri/Alien Crossfire, Diablo 2, Grand Prix2, Grand Prix4...

What game could you play for 8 hours straight?

some strategic games for sure


What game should you play for 8 hours straight?

None

What game have you played for 8 hours straight?

Ultima Online

I did loose track of time playing The Longest Journey (probably other adventure games) and Knights of The Old Republic.
Also I played Zeus more than 8 hours in a row when it was new.
I don't know about now. I don't think I could be able to play 8 hours straight. I can't even watch a full movie without pausing and doing something else for a few minutes.
I haven't tried playing 8 hours but I think something along the lines of Fallout 3 or New Vegas would keep me occupied for a long time.

At my age & life style ...none.
But two memorable titles I spent loads of time in the past:

Fallout 1 & 2
UFO 2

none as my arm would just fall off :D

but in the young days, oh, those were the times...

definitely Diablo 1 and 2, Gothic 2, System Shock 2...

What game could you play for 8 hours straight?
Strategy games most likely

What game should you play for 8 hours straight?
none

What game have you play for 8 hours straight?
X-com(old and new)
Crusader kings 2
Counter-Strike(source and GO)

I'm 100% positive there are others, but they are eluding me right now

A good RPG is played in spurts until I finish it. A phenomenal RPG is played every spare waking moment until I finish it. The latest example of this was Dark Souls: Prepare to Die Edition, that game was sooo good I played it like 10 hours a day for a week on top of college, homework, and misc. other things.

I did with Resident Evil 2 - played the entire game in one day. I think I might also have done with Call of Duty: Modern Warfare online multiplayer. Probably also with Baldurs Gate 2.

Have:
Disgaea 2
Puzzle Quest
Ocarina of Time
(I'm sure there are many others, but for these I clearly remember doing over 8 hours easily)


Could-Too many to list

What could I play for 8 hours straight?
Well, the question makes it sound like some kind of challenge or endurance test :) But I guess games I know I could comfortably lose myself in for at least 8 hours would be (and this is not an exhaustive list): Persona 3 and Persona 4. Guild Wars 2, Baldur's Gate 2.

What game should *you* play for 8 hours straight?
Like others have said, it's hard to recommend doing something as inherently unhealthy as this :) However, people should honestly experience Persona 4 at least, and a good 8 hours of solid play should give you a feeling of attachement to Yousuke, Chie and Yukiko, which would be a good start

What game(s) have I played for 8 hours straight?
Persona 3 and Persona 4, obviously :) When I was about 16 I played the original The Bard's Tale on my Commodore 64 (two double-sided tapes! The game was huge!) for 27 hours solid - bathroom breaks, sure, and I ran down to grab food and eat it in my room, but that was always at times when the next dungeon was loading, or while I was playing out the attack against 4 groups of 99 Barbarians (anyone remember that one?). I've put in multiple 8-hour+ sessions on both Mass Effect and Mass Effect 2. MMOs are a fairly easy time-sink, and Guild Wars, Guild Wars 2, and Champions Online have claimed multiple 8+ hour play sessions from me over the past few years.

In answer to questions 1 and 3, EverQuest is probably the only game I played 8 and more hours straight but I did that fairly often back in the day. I might have done that some with Morrowind and possibly Ultima VII: The Black Gate but I can't remember for sure. I certainly got sucked into both of those for hours at a time.

As for "should" play for 8 hours my answer would be none. As someone else mentioned, breaks are goodness.