My top five are not like the other ones:
1) 5.0L 307cid Oldsmobile V8 (and to a lesser extent, the 260cid 110hp version) - Only 140hp engine I can think of that can tow 5000lbs when installed in a 4000lb station wagon. Gutless but absolutely perfect for what they were.
2) 2.5L 151cid Pontiac "Iron Duke" 4-cylinder - Noisy, underpowered, but powered so many different vehicles from 1977 to 1993, including subcompact Monzas all the way up to Astro minivans, with Jeeps, AMC Eagles, and postal LLV vans in between.
3) 3.8L 231cid Buick V6 - This engine has gone through so many iterations and can be anything from an economy 80s LeSabre's base engine to a smooth grandma's Regal mid-size engine to a front drive mileage king in big LeSabres to a turbocharged monster in the Grand National to a supercharged screamer in the Regal and Park Avenue Ultra. I only wish my Silhouette had a Series II 3800 up front.
4) 3.7L 225cid Chrysler Slant Six - Actually this and the Buick six are in many others' lists, just maybe for different reasons, I have the Slant Six here because I've seen them run, SMOOTHLY, with grapefruit sized holes in the block.
5) 3.0L 183cid Ford Vulcan V6 - This powered countless Tauruses, Tempos, Rangers, Aerostars, Windstars, 1st gen Explorers, and Explorer Sports/Navajos. It may not get the best mileage, and it certainly doesn't have the most torque, but it gets the job done. Amazingly peppy with a 5-speed in an empty short wheelbase Aerostar, by the way, which is how I was introduced to this engine.
That's my list.
If I could get a RWD Vulcan to AOD adapter I want to someday put one of those Ranger/Aerostar Vulcan V6's in my Gran Torino. If that doesn't happen, I'll probably use a 3.8L Essex and 4R70W from a 99+ Mustang.
If I could have a 6th, it would be the 200cid straight six that lasted from the first Falcons and Mustangs to the last Fairmonts. 7th would be the Datsun-Nissan A-Series 4-cylinders (A12-A14-A15) from the B210, 210, and 1st gen Sentra. And finally, since I can't talk about engines without saying something about it, my 8th choice is the 1.0L Geo Metro 3-cylinder made by Suzuki. I so want to drop one of those in an '84-'87 Cavalier wagon with wide wide wide 4-speed gearing and see if I can do 49 city and 54 highway mpg, and I'll even let myself try it without AC or power steering just to be fair to the Metro XFi's heritage.