For me it would have to be in the following order:
1) WoL
I just loved the space western feel of the campaign, and of all the 3 "main" characters Raynor's arc was the most relatable and fleshed out. The missions were fun, varied, thematic and there were oodles of interesting character interactions everywhere.
2)HotS
While good chunks of the plot were wishy-washy and Kerrigan's dialogue flitted inconsistently between ragey vengeance and sad acceptance of her lot in life, the missions were by far the most interesting, and the customization options for the zerg were fun and dynamic. Also, I love Abathur to bits. Such an interesting addition to the Zerg, and his missions are far and away my favourites.
3)LotV
The "on the fly" army customization was the best option of the bunch, and the intro missions for the main campaign section was a nice bit of narrative. However I found the missions themselves to be fairly boring, most of the dialogue between missions was bland and repetitive, and I felt like too much of the story relied on narrative convenience and hitherto unrevealed factions (Purifiers and the Spear of Adun 4eg) suddenly joining the fray.
Aside:
mad825 said:
Not too sure where to start. Using the rubber stamp mark of evvviiill! on Mengsk or the constant on-the-rails/timed campaign missions which made me go back to play SC1 because at least there you can enjoy some satisfaction of defeating your enemy.
I never really got past HoTS as they so shamelessly ripped off starwars with the kerrgan/Mengsk encounter. I still never understood why kerrgan is so pissed at Mengsk, it's like "grrrrrr you've made me stronger and now I hate you, I so don't totally forgive you for that suicide mission that I accepted to undertake even though I knew it was suicide mission before hand Grrrr Ahhhhh"
I would like to make a joke but it's so totally distasteful.
Except she didn't.
Her mission was to protect the zerg hive from the Protoss long enough for Tarsonis to be overrun, and believed that Mengsk would extract her once she'd done her job. To quote that very mission in SC1:
"This is Kerrigan. We've neutralized the Protoss, but there's a wave of Zerg advancing on this position. We need immediate evac."
But Mensk didn't. He just ordered the fleet to jump, leaving her to captured by the Zerg, mutated into the Queen of Blades by an invasive and likely torturous process, then forced to kill millions of people she had once sworn to protect and save. And once she'd been freed from the Zerg brainwashing, Mengsk kept on trying to kill her and supposedly "killed" the only man she ever loved, only to revealed he'd been captured instead and would actually be killed if Kerrigan came anywhere near Mengsk.
I would consider any of these reasons a legitimate justification for Kerrigan wanting to tear Mengsk's spine out one vertebrae at a time. All of them just makes it a no-brainer
As for Mensgk being "evvviiill!", I refer you to another quote from the Terran Campaign of SC1:
"I will not be stopped. Not by you, or the Confederates, or the Protoss or anyone! I will rule this sector or see it burnt to ashes around me."
This is a man who is perfectly willing to commit any crime, perform any atrocity, so long as he comes out on top. That might as well be the textbook definition of evil. So if you're having problems with Mengsk in SC2, why weren't you having the same problems with him back in SC1? His character has not majorly changed from his previous incarnation, save he's become more drunk on his power.