I'm not sure if it was 2. I played the one where the dude was looking for his wife.... I'm assuming the wife was in the first one?

TMB I played. First TM I've played fun game but it is better suited for when friends are over. (I'm usually the guest not the host.)

Is your entire ps2 collection listed in your profile?

If so, then get these:

Ico
Shadow of the Colossus
Dark Cloud 2 (much better than the original)
Jak 1-3 (and they ARE a must)

And if you have extra cash/time:

Sly 1-3 (Excellent games. I need to actually buy them one day)
Zone of the Enders 1/2 (I'm still trying to track them down new)
The Ratchet Series
Rogue Galaxy (I bought it but have yet to play it. It's only $20 at Circuit City right now)

As for Killzone, it was a pretty good game that was marred by technical problems. The texture pop-in and repetitive sound effects were extremely annoying, and the often dimwitted AI made parts of the campaign somewhat meh. The game had excellent presentation, however (some of the best graphics on the ps2), and it had the best online play of any ps2 game, imo. The offline multiplayer with bots was also great, and kept me entertained for weeks back when I first got the game.

I'd at least rent the game. While it's technical problems can be very annoying to some, the game is quite enjoyable to others. You can check out IGN's review to get a general idea of the positives and negatives of the game.

Well I know you said you didn't want a series of games, but they have a collection of the first three DMC games for $30.  But I am not sure if it is price or time that is stopping you.

Pretty much everything mentioned so far is good, so I'll ignore those (but definitely get Ico and Shadow of the Colossus)

Zone of the Enders series
One of the Katamari games
Disgaea: Hour of Darkness
Psychonauts
Sly Cooper series
Klonoa 2

I can probably think of more later.

Edit: makingmusic beat me to a few

Its a little bit of both. Right now I'm jobless but that really doesnt matter much right now (couple months is when I'll start to panic) but its more the time I don't want to dedicate. I'm shopping cheap right now just because I need to catch up on so much.

Also there is a such thing as too much. By the time sequels have came out you would have had your fill of the previous one and havent played it in a while. (Until you hear of the sequel then you bust out the game to brush up on it. but still give you enough time in between)

Playing them consecutivly seems kinda repetitive and boring.

some others I haven't seen listed yet-

r-type final
maximo 1 and 2
fatal frame 1, 2, and 3
rygar
soul reaver 2 and 3 (defiance)
viewtiful joe 1 and 2 oh and onimusha 1 and 2 (but not so much 3)

jak 1-3 (one is not a must for story, characters do carry on, but a great game nonetheless. the second two games change up the formula quite a bit.)
sly 1-3
ratchet 1-3
kh 1/2
gow 1/2
okami
sotc
dmc 1 or 3 (story does not matter, both are excellent on their own)

coming from a guy who liked Killzone, and can't f***ing wait for Killzone 2...don't bother. It was fun back then, but now it is crap compared to such titles as CoD4 and Resistance.

Also, I suggest getting Disgaea 2:Cursed Memories, Half-Life (Co-op is pretty fun) and Disgaea: Hour of Darkness. Get Devil May Cry 1 & 3. You don't need to play 2.

If you can find them Mark of Kri and Way of the Samurai are worth looking into. They won't blow you away, but they're pretty cheap so you'll get your money's worth.

all the tenchu games were good