For my comment we have to cover a few of the basics.
Halo - While personally I greatly loved the first game and ODST the others just became simpler shooters. The game seemed to lose some of the difficulty, not sure if this is due to them making it easier or just myself getting used to playing the game with my toes. None the less a game I can go through on the hardest difficulty with my feet doesn't speak much to it's own merit. Halo had a nice story progression but we have some interesting flaws cause by the games outside the core 3. None the less a rich content of books, militant like fan base, and causing tea-bagging on a world wide scale has caused many, young to almost middle-aged alike, to call this one of the greatest franchises. This being said I feel that they didn't bring enough challenge to the players in the campaign so this while deserving mention, doesn't win my vote.
Fallout - I remember playing the original title before I could properly grasp what is was I was doing. All I knew is I loved stacking the sniper perk about 4 times and shooting every in the eyes and laughing extra hard if while blind they happened to shoot and kill someone for me. I would raid everything because I felt that everything in the game lawfully belonged to me since I was the only thing capable of free will. As I got older and continued to play I realized that there was a rich story to be gained by not treating everything in the game as "walking treasure chests of happy fun-time and bewilderment" but instead to truly immerse yourself into. both Fallout and Fallout 2 had a wonderfully rich experience to behold with no real direction you "had" to follow. Half of the fun of it was what you could come across in random encounters with the occasional more fan serviced encounters such as the "Smite the Lamer" riot where a whole collection of people ran up to beat the tar out of a man only speaking in "L337" or even the reenacting of the bridge-keeper scene from Monty Python and the Holy Grail where instead of a fall to the bottom of the chasm, a gruesome death by Gatling gun awaited the one who answered wrong. As the serious improved in graphics it lost some of that freedom and random moments of fun to take a more serious tone with Fallout Tactics. While game play improved it sacrificed the ability to take things in your own way of going through the game and regrettable this style carried on to the Xbox 360 titles. I feel Fallout had a more fulfilling experience when every aspect of the game was up to you on how and when you did it. The conversion of this environment into a 3D world was still a wonderful transformation where there was many things to do to the side but there was that always overwhelming feeling of your "main quest" trying to hold your hand and make sure you don't get lost in the woods because we don't want people without a sense of "whats next".
Half-Life - It's hard to review something that opened my eyes to such a well done and dynamic experience. No matter where you are in the story, you feel compelled to the very end to push forward. Not because you have to, this is an action of want. Half-Life gave people like me the ability to read a good book without looking at too many words in a row and getting sleepy, not to mention John Woo always had your back with some well times hand grenades to, everyone's favorite video game tool, red explosive barrels. In real life you look at one and think . . . "I wonder how this would look if it exploded." For video games there was no wonder. We would often challenge ourselves to see how much destruction we could cause, from kills to domino effect, we will always have a fond place in our hearts for them. But ultimately Half-Life can be easily considered great in my book for the fact of it's relevance to modern society. This is something most of us have though about as a real feasible outcome. Not to an exact recreation but our own fantasies each let us see what we want. Because of that I feel that we can relate more to Gordon and his vigilante, crowbar-wielding ways. I am sure I am not along in giving this one high praise.
Starcraft - This alone could get hours of attention but when it's not an original idea it just seems to be lost to me. I give the game-play a solid 9 but every time I look at the terrains I see less impressive SPACE MARINES! which will bring us into the final point of my rant.
Space Marine - It only took 24 years to finally get a decent shooter out if the series. The campaign well constructed with some fantastic over to top moment from the beginning till end. It was a wonderful joy ride to behold and had a smile on my face for almost all of it. *personal note: the only other emotion displayed was RAGE!!!* With it's incredible rich lore and almost 25 years of books, this series has the most invested time out of any other. The main thing holding it back is it's poor net coding for multiplayer. I would have to give it 2nd place in the Sci-Fi category.
In my opinion Half-Life gives me the best there is to offer while still maintaining a reasonable connection to our current world.