I've played all three. Portal 2 was amazing. I fell in love with Portal when I played it on the Orange Box. So much so, that I didn't bother playing either of the other two games in that thing. Portal 2 was amazing. The only reason it didn't surpass the original is because the original was just THAT good and THAT novel. It did, in my opinion, match it however.
Deus Ex: HR. Great game. I had a fantastic time playing it, thoroughly enjoyed the worlds. I started playing it about 1.5 weeks before Skyrim came out. I *almost* finished it. Then I put it down, because I had Skyrim. I was seriously within 1-2 hours of completing the game, and I put it down, and I haven't felt the need to pick it back up. I've put in well over 200 hours into Skyrim now, and I still don't need to finish Deus Ex: HR yet. I believe that I will be putting another 100 hours into Skyrim at least before I need to look at that. I bought the latest AC game too. Haven't touched it. Only have it because I pre-ordered it. Neither myself, nor my wife have any pressing need to play that either. We have Skyrim to save and explore, guilds to crash (read: become the leader of) and dragons to pet (read: kill).
The real question, for me is this: When you've completed the game, did you feel the need to play MORE?
Deus Ex: HR (grant, it's only *almost* done) No. It was good, but I wouldn't really want to try to play with another "type". Avoiding detection entirely was cool once, having completed a particular mission once before and repeating it, I was able to avoid detection. But the idea of trying to do that for all of them? Damn, no... I'm good, thanks.
Portal 2: Nope. I haven't touched it since I finished it other than to show a friend. Great game and all, and I took my time with it, messing around with all the various "tools" in the game and it took me about 7-10 hours. I explored the hell out of the limited world it gave you too. Great fun, don't need to play it again any time soon. I also don't really do online gaming, so all of those levels were lost on me.
Skyrim: Hell, I didn't even finish and I restarted. I had 170 hours in my first character, already better than 30 in the new one. Outside of the start-up quests, we've done (I play with my wife) a crap ton of stuff that we hadn't done previously. I'll be playing Skyrim well into the new year. Eventulaly I'll get around to finishing Deus Ex and AC...whatever it is right now, Revelations. But I'm not worried about when.
Right now, I only have eyes for Skyrim

Edit: I'm not fixing that... but wtf? Why did all that html end up in my post? I typed in notepad, zero formatting, then pasted here (I'm at work at it's less conspicuous than the black background on the forum.)