First and foremost - the solo campaign is probably the most iconic in the Warcraft franchise and you get all the good juicy parts. The story of Arthas, the rise of the scourge, the night elf/orc conflict, introduction of various popular characters. The campaigns can be both hard and easy and will vary throughout the campaign. It isn't all about building up a town, produce an army and complete objective.
When you say solo/AI games, I think the value lies is within the modding scene. People who love the game have created a giant cluster of custom games that have dictated gaming in million ways. You have TDs, survival games, strategy games, hero defences, tag games, minigames, roleplaying games, hero arenas, RPGs, and some of these you can play in solo or with friends.
If you like the Warcraft story, then there are various spin-offs stories where some have implemented the wow story and others, where modders themselves have designed the story and created it. (Custom Campaigns)
In my opinion (with consideration you don't want to play ladder), I think Warcraft 3 is the most fun when you play with friends. (LAN)