Master of Orion 1 & 2, Civilization series, HoMM 1, 2, 3, Age of Wonders 1 & 2.
These are the bigshots of the genre.
Multiplayer doesn't make a bad game good. But a lack of multiplayer makes a good game not worth playing. The social interaction of a 4x is a huge component. It's like a better boardgame.
Here's verbatim messages I sent to my 2 cousins and my brother (collectively, we've played probably 100 games of AoW 1 & 2 each, all multiplayer):
"Hey, I found a great looking game like AoW, Fallen Enchantress: Legendary Heroes. Looks really good"
"Nevermind, there's no multiplayer."
"Shitty, that was one of the best parts."