That is a difficult question. I haven't played Dawn of War 1 in a long time, but that's largely because I can't figure out how to get my old mod to work, and because I'm an eye candy junkie. Back in the day I probably played it more than I've played any of the 2s, and I'd say I enjoyed it more too (particularly 1 and Winter Assault, plus Dark Crusade's skirmishes). It just feels more Warhammery, with the larger scale and more races/units, and personally I enjoy base defence-type games (which is by no means what DoW was ever intended to be, but the bones are there in 1, which I fleshed out with modding tools).
But, I'm also hugely fond of 2's Last Stand gametype, which is only playable online. You control a single 'hero', along with two other players, and together face down progressively tougher waves of AI enemies in a small-ish map. The trick, aside from map and enemy knowledge, is to pick an equipment loadout that'll get the job done, and work best with your teammates. It's entirely co-op, and you can win the map in the 20th wave (about half an hour's play I think) if you're very good.
They actually made Last Stand purchasable as a stand-alone mini-game, as it's so different to the main platform, but if you buy it you're just getting DoW2 with most of the functionality unavailable.
Tabletop Warhammer in general has gotten insanely expensive in recent years, so yeah, probably better off playing games based on it. It'll save you going blind painting the tiny little dudes, too.