I played all 3. Halo is one of my favorite franchises of all time, COD is decent and I played the Orange Box and half life...I just don't like it. I can't put my finger on it, it doesn't really feel like an alien invasion, just a generic war shooter with some really weird monsters. I don't like stopping every five minutes to stack up some boxes, solve a stupid forced puzzle or just figure out where the Hell I'm supposed to go. Also the vehicle controls are just crap and for the love of god they're an eyesore to look at, remind me why they had to tear them down to the bare skeleton again? Also that boat section at the beginning of HL2 was TOO FREAKING LONG and it was a tease "Oh you can stop now and get off and-HA just kidding, you just removed an obstruction." Also the architecture just bugs me too, it's too blocky, too dirty for no reason and half of the time it just feels out of place, Combine architecture in particular rubs me the wrong way. Also the Combine feels very underwhelming as an enemy, I just never felt like they were a threat, also that rocket launcher left something to be desired. I took a peak at Episode two when I was almost done with HF2 and when I saw that the controls and graphics and...everything else was the same and had been released over the years I gave up. Also, this amazing story that I heard so much about, frankly I found it lacking, it's just a bunch of people worshiping Gordon Freemen for some reason I could never really fathom, he's just a scientist, not a super soldier like Master Chief or a decorated veteran like Command Shepard or Marcus Fenix. I really have no idea why that franchise is so popular to me it's EXTREMELY overrated, then again people attack Halo a lot so eye for an eye.
So...yeah.