If you want the best, purest RTS experience, get Dawn of War 1 or Dark Crusade.
If you want the most advanced RTS with, as was said, the most "bells and whistles" Dawn of War 2. The SP campaign is basically a lite-RPG, which sets it completely apart from DoW 1. On the other hand, I found the actual battle portion of DoW 2 much less fun than DoW 1. Way fewer units, and a sort of "hack and slash" mentality. I could write pages on what I think is wrong with DoW 2, but I'll just say it can be fun if you're not expecting a classic RTS experience. It also has a meta campaign that you can play through DoW 2 into Chaos Rising.
Just can't get over how boring and repetitive the SP missions actually are.
Agreed on the "fun if you're not expecting a classic RTS experience" thing, for certain. DoWII is built off the same engine as Company of Heroes, if I'm not mistaken, and as such it plays
nothing like a classic RTS. Heck, in the first two single-player campaigns there wasn't any resource gathering or unit/base building,
period (and there basically wasn't any in Retribution, either). Rather, it focuses on small-scale, squad-based combat, focusing on use of cover and suppression to control damage and mobility.
Online play continues the CoH tradition of controlling territory (via control points) to gain resources, and while you now get to build units there's still no base building.
So, yeah, I should probably confess that DoWII arguably isn't an RTS at all - it's more of an RTT, a Real-Time Tactics game. Or something.
A real RTS needs height advantage and bullets that can't travel through 300 feet of solid concrete/metal/rock/dirt
Now THAT is classic RTS material, right there.
I'd like to point out that DoWII has both height advantage
and bullet-obstructing terrain, along with flat-out
destructible terrain (which matters a lot when said terrain provides valuable cover!).