I have played far too many games over the years to give a really accurate list of just 10 games.
What I will do is list the 10 games that have had the most influence on me as a gamer (thus most hours spent that I can remember right now)
Nr:1 - Oblivion (I prefer this over Morrowind even if just slightly and also it beats Skyrim by miles due to content and story (the guilds, secondary quests)) I never really play the main storyline I always play every guild, secondary quest and dungeon in the game. Create a new character rinse and repeat. I only ever completed Oblivion once & that was in 2012
Nr2. Command & Conquer. This is between Tiberian Sun, Red Alert 2 & Generals (incl Zero Hour) don't make me decide
Nr3. Half Life (1,Opposing Force, Blue Shift, 2 - they are all good)
Nr4. Counterstrike
Nr5. Lord of the Rings The Battle for Middle Earth 2 (the hours of fun in LAN games on this game are some great memories I treasure)
Nr6. Medal of Honour
Nr.7 Call of Duty 2 (no more no less) , C.o.D. 1's single player storyline was also one of the best it gave you a sense of Enemy at the Gates
Nr.8 Rome Total War - It was a brilliant game, I hope Rome 2 redeems what Shogun 2 has shunned (Shogun 1 was far superior to Shogun 2)
Nr.9 Fable - The lost Chapter (again no more no less) after the fail that was Fable 2 I didn't even buy nr3 I still want to but I am waiting for steam to sell it for -75%
Nr.10 (This one I will leave up for debate
😉 ) <- scratch this I will say Bioshock Infinite due to the amazing storytelling it had. you really feel like Dewitt