I'm a motor head. my list revolves around significant advances in design that led me to where I am today. In other words, I don't care about diesels, wankels, boxers, or any acheivement from the company that brought us the Gran Turismo Olemagato.
So My gearhead list (In order)
Ford flat head. original hot rod V8. A flat head was the springboard for the "hobby" we know as hot-rodding. They were the 5.0 of their day, and the aftermarket still builds junk for these things.
The 265 SBC. The engine basically remained the same for 50 years, until the debut of the LS. The Bane of my existence for all of my adult life.
The Chrysler hemi. Starting as a lowly 358? or some crap like that, the engine basically "morphed" into the 426, and became the premier engine in both NHRA, and NASCAR. It is still the basic platform for all things TF/FC.
The 427 Cammer or SOHC. Fords original attempt at a hemi. (only w/ dual OHC's activated by a 6' long timing chain) Ford rated it's output at 615 hp. NASCAR outlawed the engine after it started killing everybody else. It spent the next several years in the NHRA helping factory ford racers keep up ( and beat) the Dodge boys.
The BBC. Introduced in the middle 60's as a 396 ( I still love saying the numbers) That engine came in all sort of evil configurations until It ended up being the highest rated production HP you could buy in 1970 in the form of the 454 SS chevelle.
The "Shotgun" or Boss 429. Offered in both a street version, and a race version, This engine came closer to being a true hemi, but still lacked the hemispherical combustion chambers like the chrysler. It was called a "semi hemi" instead. It was significant because it was made in a street version to allow Ford to play in NASCAR w/ the race version. It wasn't outlawed like the SOHC, and Ford finally had an engine they could hand chrysler's ass to.
The Boss 302. Again built as a street version so they could play in the trans-am circuit. The intake ports were big enough to literally get your hand into, and as a street car it was a lousy driver because of that. But once you got it past 4500 RPM.......
The 5.0. Wouldn't be what it is today if the aftermarket hadn't got behind it the way they did. After suffering for 30 years as a 260/289/302/351W with chicken-dick stock heads, World products was the first to finally give the little engine a new lease on life. And a way for Ford racers to finally beat that chevy ass.
The LS-1 Still a pushrod engine but w/ better design all around and durability to match.
The 4cam Mod motor. For better or worse, ( at least in this forum) Probably fords last V8. the platform that will see Ford through the end of "no replacement for displacement" where turbo/forced induction 6's and 4's will become the way of the future.
Thats 10, if you asked for 20 engines, some pontiac and Buick junk would've probably thrown in as well.