Disclaimer: I'm approximately 788% biased towards the WH40K universe, particularly the 'originals' as I refer to them (the RTS games, that is).
Dawn of War, the original, was amazing from the start. Superb music (Jemery Soule), great voice acting ("SPAAACE MAAAAAWHREEEEEN!!!", fantastic VA for Gabriel Angelos by Alec Newman). Plus, that original CGI intro they made for that game is a classic one (but I have to say I really, REALLY liked the CGI intro for Dawn of War 2 as well).
Winter Assault, as well, was about just as good, and featured the Imperial Guards; what's not to like? Also, for that one, the OST was composed by none other than Inon Zur. It was also a good foreshadowing for the arrival of the Necrons that would then be featured in the following expansion. It also had a solid, well done campaign.
And then the ultimate happened, called Dark Crusade. Now here's the thing, I am a HUGE mega Necrons (and Craftworld Eldar) fan. They're in my top 3 factions of the whole 40K universe. So yes, I screamed like a little schoolgirl when I first heard of the news about that expansion. Little did I know that it would end up being the absolute best overall of the original 4 RTS games (DoW, WA, DC and SS). I could go on and on about why it's so good, but I think it's better explained by simply stating that it is FUCKING good.
Now, when it comes to skirmishes and just pure number of factions to choose from for diversity's sake, then I'd have to go with Soulstorm on that one. Not for the campaign, but merely for doing skirmishes. There's a community unofficial patch mod that helps fix a good number of issues that were never addressed by the devs at the time (Soulstorm was rushed, was rather unpolished when it came to factions balancing and had some bugs that shouldn't have passed any internal testing). For a good Soulstorm experience, I recommend, nay; I HIGHLY recommend to download 3 things: 1) The Soulstorm Bugfix Mod, which I believe is at around version 1.56 or something along the lines (I haven't chekced in a while if it was updated further), 2) The Dawn of Skirmish A.I. Soulstorm Mod (which... may actually have been included in the Soulstorm Bugfix Mod package, that too I haven't checked in a while; I always just downloaded both separately by habit), 3) The Dawn of War Map Packs, four of them I believe, all working with Soulstorm (even though last time I checked even if the maps do work in the game and load properly, are missing some map preview icons / thumbnails in the list of maps when setting up a skirmish).
The pinnacle RTS Dawn of War game though, with a proper campaign and the whole overall package of quality is, without a doubt, Dark Crusade for me. But I do go with Soulstorm for skirmishes.
Oh and, yes, I almost forgot to mention this but I do consider the original DOW RTS series (all four of them, overall) to be my #1 favorite RTS series of all time (with an extremely close 2nd going to Command & Conquer, and 3rd to the original StarCraft + Brood War, and I'd definitely give honorable mentions to Company of Heroes, Supreme Commander, Age of Empires 2, Rise of Nations and Warzone 2100).