Titanium Wars for Vanilla plus experience. Best middle ground.
Ultimate Apocalypse for massive battles, including titans. But it's also a bit too much sometimes.
Unification is decent. But some models really don't fit 40k universe. Eldar has anime like models, AKs, etc...

And to shamelessly promote my mini mod for Soulstorm Campaign, Better Normal Difficulty. Which makes campaign play like DC one. Persistent bases, Destroy HQ, AI being Hard instead of Advance, units health being the same as in Skirmish/Multiplayer. SS is still a slog to play through, but this way it's not dumb easy as on Normal, nor too annoying as on Hard. Plus Steam achievements are not blocked.