Hmmmm. Just off the top of my head:
Omaha Beach from Medal of Honor: Allied Assault. Will always be awesome.
The Heat-inspired bank heist mission from GTAIV. Makes me super excited for the heists in V.
As far as bosses go, pretty much all of Dark Souls.
04/16 - Ringworm
04/19 - Trog
04/22 - Ghost
04/24 - Iron Reagan
05/13 - SLAYER