Out of the ones in the poll - STALKER. Otherwise I like Slavjank squad-based tactical RPGs the most. Silent Storm was already mentioned but there's also E5 New Jagged Union (very subtle with the game that inspired them) and its sequel 7.62 High Calibre, though admittedly I have only played the Hard Life release of 7.62 which is basically the base game but with the fan-made mod Hard Life already tacked on it.
Its only glaring problem is that it makes STALKER seem stable by comparison. I remember I had to save all the time not because I wanted to save scum but because in certain sessions I legitimately had the game crash every 5 minutes. It's a black mark on an otherwise insanely detailed tactical RPG. In most games of its genre the only relevant thing when it comes to whether you'll hit someone or not are trooper's and his gun's stats and whether the enemy is in cover. In 7.62 you have to deal with the trooper's heartrate, stress, injury, whether the gun is balanced or not (for example if you put too many attachments near to the barrel that affects how fast they aim and how they control recoil), what kind of helmet they're wearing, whether they have poor eyesight or not (there's even ways to correct it in-game, from wearing glasses to paying for laser surgery) and many more. Another crazy detailed thing is that ammo doesn't just get differentiated by type (as in FMJ as opposed to HP), but by manufacturer, so a cheaply made 5.56 round will lead to more misfires, jams and will deteriorate the weapon faster. The story isn't anything to write home about and the quests themselves have the writing quality of MMORPG quests but there's actually a lot of player freedom when it comes to joining factions and completing certain missions and what not.
Another series I remember liking was the UFO trilogy. I remember the least about Afterlight and I played Aftershock the most, though ironically I've never beaten Aftershock once. There is actually a game-breaking bug that makes a piece of research required to continue the game just not appear and back in the day you had to send the savefile to the devs so they could fix it but I never even reached that part of the game, honestly the game becomes a bit too samey and boring and I always stopped playing after the Starghosts arrive. It's not that they're hard or anything but at that point your troopers are so cracked that there's no real challenge anymore. Ironically, seeing just how insanely OP your troopers become is what makes the game fun too. I think pretty much every type of specialist can become OP as fuck, but I remember having the most fun with Rangers, Stalkers and Snipers. The former two are close ranged specialists, the Ranger focusing on guns while the Stalker focuses on melee weapons. They both become insanely fast, there's nothing like seeing your Ranger clear out a room full of aliens by rushing into it at breakneck speed and burst fire 3 aliens to death with dual-wielded MP5s before the aliens can even raise their weapons.
