Orange Box, Orange Box, orange Box!
- You get the whole lot of Half Life 2, which is in itself some 50 hours of fun to be had (and in its way, a lot like L4D - repetitive, but fun XD).
- You get the best single-player game ever made with Portal, and even if it's only 3-ish hours of fun, it's the best ride You'll ever have!
- You get TF2, which is the best online FPPS I've ever played, period. Yes, I rank it even above gems like Quake1. I got my Orange Box more than half a Year ago, and I still play more TF2 than any other game I have ( didn't even play through the Witcher yet, even though I'm a fan of Sapkowski. TF2 grabs all the attention XD ). A bit of a steep learning curve for the first week or someting, but great fun nonetheless.
- As You said, it's 3 games ( I don't count HL2, HL2.3 and HL2.6 as separate games XD ) for way less than L4D costs.
- You get 4-people-Co-op with L4D. There's 6-to-16-People-Co-op in TF2. And You get more weapons, more classes, more maps, more unlocks, more everything.
-> definitely Orange Box, and wait for L4D price to drop.
Edit: Hm, after reading through, it seems half of my points are nil and void XD Well, let's say it this way:
TF2 alone is way more fun than l4d. Yes, it doesn't have a single player mode, but then again, L4D is not worth the price for single player mode, and multiplayer mode just pales in comparison with TF2.
Maybe You could consider not buying neither, but just waiting for a price drop on TF2 on Steam? We'll be getting a scout update in january, which usually means either a free TF2 weekend for everyone, or a price cut for a limited number of days. Would be Your opportunity to either test it, or to get it for $10 or something along those lines.
HL2:Ep2 Is rather short, and it does not conclude the HL2 storyline (open ending again), so... not really worth it at the moment, I'd say, if You already played HL2 and Ep1.
~Sylv