Nice too see all the users' gaming options, might have to try some of those that you guys have mentioned so far.
For me:
Tactical Strategy: Total War Series especially Rome II and Napoleon, love fielding thousands of troops and the realism with morale, ie fear etc, troops running away from battle and so on. WW2 or modern equivalent to it for me would be the old Close Combat Series especially IV/V, first game I saw where enemy troops actually surrender and vehicles could break down/run out of fuel, and these days Men of War - Assault Squad 2 and Call to Arms. For space age strategy I love Homeworld 2 and it’s Star Wars mod.
Grand Strategy: Probably Hearts of Iron IV (WW2), Guess Total War also fits here as it's a hybrid game series.
Semi Realistic FPS: Squad, Red Orchestra 2 (WW2 Eastern Front), Rising Storm Vietnam after trying those, Battlefield series now seems too cartoonish for me, ie no shell deflection mechanic in tank to tank combat and the combat in general. Still not bad either though. For twichfest type games I'd say Unreal Tournament or Star Wars Battlefront 2 (2005) both of which had tons of amazing mods, were my favorites. My first foray into FPS was probably Medal of Honor on Sony Playstation.
@Custom90gt, I too spent countless hours on BFII lol in college, along side Battlefront II, Red Orchestra (at the time a UT2K4 mod) lol. AOE1/2 were also childhood favs, before I discovered Total War.