The banshee in Halo: Reach, especially on the levels "Paradiso" and the slightly altered version, "Utopia." Imagine an armored vehicle that can fly and shoots an unlimited amount of extremely overpowered rockets and can fly faster than any land vehicle and can do impossible flips and barrel rolls that make it nearly impossible to hit with any weapon and causes any projectiles that are locked on to it to miss.
Now imagine that same overpowered flying death machine on a small, cramped level with no hiding spots beyond a single cave in the middle of the map that is impossible to get to because you only spawn in a specific area of the map and getting to it when the overpowered flying machine is firing its overpowered rockets that can kill you if it hits anywhere within a 30ft radius, including on the other side of sturdy rocks, and the ground surrounding your base is only about 100 square feet.
Also the Shadow Operative skill from Alpha Protocol that literally makes you invisible as long as you don't sprint. You can walk straight up to a group of enemies and "stealth" kill all three of them in front of each other and they have no idea why their friends are dropping with knife wounds on their necks.