This is just my opinion.
5: Knight - Stronghold
The knight, or it foot counter part the swordsman, are by far the strongest unit in Stronghold. Able to take tons of damage and keep on ticking, it is able to storm castles with ease. In Stronghold 1, with +5 morale added to the knight, it give it an additional 50% health, making it even stronger. It is so strong, that sometimes, the only thing that can kill it is a catapult or fire. In Stronghold 2, they made the knight extremely strong. 3-5 knights could defeat an entire castle.
4: Mammoth Tank - Command and Conquer
This monster of a tank has 4 tracks and 2 guns. The Mammoth tank appears in almost all C&C games, under several names like Apocalypse tank and Overlord tank. It can deal just as much damage as it can take, which is a lot. Some models even have missiles, that can fire at air units, making it effective everywhere. The strong model is the Emperor Overlord tank, found in C&C Generals: Zero Hour. The EO has a Gatling cannon, loud speaker, and, as well as the standard guns, has a BUNKER! Yes, it is so large, that it is a moving bunker.
3: Thor Tiger - Outpost 2
I'm pretty sure this one is self explained.
2: Battlecruiser - Starcraft
While the last three we ground units, this one is an air unit. The Battlecruiser is know as one of the icons for Starcraft. It comes into play late in the game, and can easily be a game ender. Having crazy fire power, the Battlecruiser is able to destroy bases with ease. Careful management of the BC can make it very effective.
1: Fatboy - Supreme Commander
Ripping bases apart has never been so fun. Supreme Commander had alot of powerful stuff, like nukes and 1000 pop cap, but nothing was as cool as the Fatboy. The Fatboy was a mobile factory, meaning it could make units and it could move. It had 16 tracks, and 4 battleship size cannons on it, yet it was a land unit. It also had a helipad, shield, anti-air and could be submerged. It was a killer. There was no unit more powerful then it. In a 1v1 battle with any other unit, it could make units to help it defeat its foes.
So, any suggestions on any better units?