This is the best list in the thread, though I'd replace RE4 with Uncharted 2 and add Red Dead Redemption. If you want to build a list of games that best represent the medium, then this is it.I don't even think older games are "essential" any more. While they influenced many new games, it's hard to see that in some cases and some of the control issues are infuriating.
If I was to make a list of modern (or have a modern release) games, where you can clearly see influence throughout the industry, I'd probably say something like:
COD 4
Mass Effect
OoT
Skyrim
DOTA 2
Final Fantasy 7
Halo CE
Team Fortress
Resident Evil 4
I don't even like most of those on that list, but they're all kind of land mark games that you can still see, feel and hear the influence of.
Some niche titles may be your personal favorites and there are many games that are much better than any on this list but these are by far the biggest influences on the medium thus far and therefore deserve to be called the essentials.