Damn, so many games have made me go "10/10 BEST FUCKING THING EVER", so I guess I'm gonna have to narrow it down to genre's then.
FPS: Okay, I'm cheating a little here but I gotta put two here. First place goes to Half Life 1 and it's expansions, because by god does creeping around the Black Mesa facility make me feel like a nerdier version of John Mcclane, gunning down alien scum and power armored soldiers like nothing. It's even better if your playing as Barney in Blue Shift, or (IMO) realest nigga Adrian Shepard, what with the new toys to play around with and new baddies to face off against.
Second place goes to System Shock 2, because being berated by cyber-waifu goddess SHODAN makes my pp hard uwu. But for reals, the atmosphere and many, MANY ways to play through the game is just top notch. Not to mention the overarching plot and excellent pacing, to say nothing of the awesome voice work and characterization. Also, MARINE CLASS ALL THE WAY, CRAYON MUNCHERS UNITE!
I would legit take SHODAN's offer at the end of the game, her being the mission control was so fun and interesting.
RTS: DOW: Dark Crusade. Hands fucking down. SPACE MARINE MAIN 4 LYFE NIGGA. Slaughtering Eldar/Tau fags by the hundred by the dozen with permenant corpses and a buddy or two is so fucking fun I tell you.
Survival Horror: Silent Hill 2. There's nothing better than going into the game blind and screaming your ass off at all the mind warping horrors in that hellish town. Great fun.
Stealth: Thief 1, straight up. Thief 2 is awesome too, but I liked the more supernatural horrors more in the first than the technological uprising in 2. That's just my preferences tho, both are just amazing little specimens to play through.
RPG: Can I put System Shock 2 here again? Fuck it, I'll put it here again. I would have put Bioshock 1 here, but I thought it through and realized it's not much of an RPG, more an FPS with RPG mechanics.
Mech Sim: Mech Warrior 4, with the addons installed. So many options, so little time. I actually got emotionally attached to my teammates, something only Spec Ops and XCOM did to me. Speaking of Spec Ops...
TPS: Spec Ops: The Line. By god, I was weeping like a little faggot by the time the game ended. I will NEVER look at sandblasted cities the same after what I went through in that game.
So then, what's the WORST game I played? One answer to this one, and one only.
Aliens: Colonial Marines.
So, I have some history for this one.
Long ago as a little boi, my public school had a magazine area in it's library, and naturally as a kid obsessed with vidya I immedietly read through every gaming magazine there. I saw Halo 3, Gears of War 2, and all this crazy shit my little mind was so hyped up for. But then I saw a specific title, one that made my heart skip multiple beats; an Aliens FPS, with a focus on survival and squad management.
Thing was, I was (and still am) a massive Aliens and Predator fan, and AVP2 is one of my favourite FPS games to come out of my childhood, so when I found out about this glorious new FPS being made by the fucking Opposing Force devs, I was practically nutting in my size 5 shorts. I saw the release date, blabbed about the game to my parents and friends, and waited.
And waited.
And waited.
Time passes by. New vidya comes along that I enjoy, and I have fun only a kid could back then. But always at the back of my mind, A

M was there, like a Xenomorph waiting to strike at any moment. Occasionally I would hear things about the game, but as always I would get hyped for a while, and then it would return to my subconscious, just waiting for that pivotal moment.
Then when a solid release date came to by, I commited an infernal sin I shall never forget for the rest of my living days.
God help me, I PRE-ORDERED the fucking game.
I was naive. I didn't realize the history behind the devs, or Randy, or even realized that these were the same people that shat out Duke Nukem Forever. I just mindlessly pre-ordered the game and waited for the promised time.
And it came. I remember clearly the conversation me and my dad had when we were driving home, him saying he saw reviews that damned the game and that I might want to reconsider my choice. But I was desperate, I couldn't wait any longer. I refused to refund my purchase, and when the time came, I played the game.
After 3 hours, I quit, and actually cried to my dad to somehow refund the game. This was at the time when Steam refunds weren't even a thing. Naturally, my father refused, saying this was a rightful lesson to be learnt, to not literally buy into the hype. Of course I freaked the fuck out and that nailed me three weeks without the computer.
To this day, I stare at that cursed icon on my steam library and think to myself, "Damn you Randy. Damn you for taking both my money and my childhood innocence". I will NEVER, EVER forget my experience with this game, and to that end I have to say with all due frankness, it is the WORST fucking thing I have ever played.
TLDR; Aliens: Colonial Marines made my 12 year old self meltdown and cry like a little bitch because my money was robbed by Randy "The Great Satan" Pitchford. For that, his game gets the noose.