Half Life 2 would be mine.

Re4 on top. Very happy with the result.

It looks like I need to add Resident Evil 4 to my list of games to play before I die. Which would you guys recommend between the PS2 version and the Wii version?

Wii, for sure.  It controls great with the pointer and looks better.  I think you get the extra content from the PS2 version as well.

If those are your only 2 options then the Wii version is by far the version to get.  It has all the extras that were added to the PS2 version like Separate Ways.  It doesn't have the graphical downgrades which were needed to get the game to run on the PS2 and the cutscenes are in real-time rather than video files.  It has the far superior pointer controls and the option to use the Gamecube controller instead, which is still better for the game than the PS2 controller since the game was originally designed around the Gamecube controller.

Even (modern) integrated PC graphics could probably get you through the UHD PC version with the now-essential texture mod.

Half Life 2 is a good candidate too now that I think about it... Still, I feel Halo CE basically made FPS the dominant genre for the decade and it preceded HL2 by three years or so... Not only that, it put XBox on the map.