I prefer Halo for it's fair & competitive multiplayer; all it needs now is dedicated servers and a mix of the last three games. The storyline is exotic to your classic space marine/modern military shooter; and it doesn't hide from the fact that you ARE better than every other character, you will kill hundreds and hundreds and in the third game your opponents will herald you as a demon of their kind whilst your species hails you as a god. It was just good.
Half Life is a close second. As one thread was asking, while originality is important with new games it is not critical. Regardless, HL plays it hand well when it comes to originality. You're a scientist. There are aliens. A deep underlying story ensues following the classic "survive invasion, thwart the bad guys" cliché; but the style of play and the way they tell the storyline is just completely unique. Even without that it is just as good an adventure/FPS/simple puzzler game - it is my close second favourite for this reason.
CoD... *SIGH* No point wasting 1000 words over something I've critically disliked since the 5th "adaptation" for a number of planet-sized reasons of which, for some weird reason, are still recurring faults that pop up in every single game of the series. Remove the title "CoD" and change the graphics slightly and I don't think the game would sell well at all; and I also think Activision know that. Treyarch nearly fixed Black Ops (minus 1 or 2 series flaws such as hit detection and awful maps) and it's now one of the least popular in the series, despite selling second best. Take CoD 1, 4 and 7 out of the equation and you've got a dump of a series.[footnote]
Yeah, sure Halo has been milked just as much, but the demand is still there and the hate, not so much. When Bungie added a few features then sold it off as a another game, they labelled it as "budget" themselves due to the lack of fresh content - when Activision pull the same stunt with CoD - it's more-than full price each time.[/footnote]
Off-Topic
CoD was good, then over-popularity happened; and things turned greedy and now we have a poor, weak, recycled game series - now they're trying to set up subscription plans.
Guitar Hero wasgood, then over-popularity happened; then things turned greedy and then we had a poor, weak, recycled game series - then it got axed when consumers caught wind.
Tony Hawks was good, then over-popularity happened; then things turned greedy and then we had a poor, weak, recycled game series - which got shelved indefinitely when consumers caught wind...
Seeing a pattern yet?