Well seems you've already made up your mind but "quick" overview anyways:
Dow 1:
Have nostalgia for the original vanilla campaign and serves as an effective introduction to the setting and dow games in general.
If you'd rather skip this campaign I recommend at least checking out BigDickCheney's dow campaign cutscenes "remaster" to be caught up on memes (SINDRIIIIIII), important recurring or notable characters (Gabriel angelos/ Farseer Macha) and general plot since later dow games build on events of this 1st campaign.
Ex:
https://www.youtube.com/watch?v=8tfJ2a7wD24
Expansions are Dark Crusade and Soulstorm.
Dark crusade has probably the best campaign overall simply because bases and constructions you build in missions are mostly permanent..Aka if you fight on desert tile X of the overworld map and play an hour long mission and build a sprawling base, then later on fight on that same tile again, map used will be the same and your big base will still be there. Not many rts games had a campaign like this.
Soulstorm lacks this innovation which is why it's an automatically inferior campaign, on top of any questionable design decisions which makes the campaign subpar even though there are more races. Still, you'll want soulstorm if only for sisters of battle and dark elder in mp.
Finally the real reason you'll want dow1 and its expansions: the modding scene is phenomenal, it's the reason I've stuck with dow1 for almost two decades now.
From more "competitive" focused mods like dowpro (basically random ppls attempt to balance out vanilla whilst introducing more fluffy choices) to ones that add minor sub factions or full on races (imperial guard steel legion, tyranids, inquisition, chapter focused overhauls for blood angels, salamanders, etc) to these crazy big overhaul mods (ultimate apocalypse throws in the kitchensink, even has titans ffs long before dow3 did it). Seriously it's the skyrim of rts games when it comes to mods.
Dow 2:
Shifts from big rts battles to a more squad based tactics game. I hated this in mp and is main reason I stuck with dow1 and never really got into dow2 in the same way. But I still had some fun with the campaign which has a more rpg/diablo feel. Each of your predefined squads is led by a leader character who you level up and spend stats on and give them gear you loot from missions. Makes for a different sort of campaign experience and is enjoyable enough. Oh and its coop too so that's pretty cool. But yeah mp wasn't my groove in dow2.
Chaos rising expansion:
Chaos rising appropriately adds chaos in mp and continues the story of previous campaign with same characters, and besides each level being individually better designed and overall meatier then levels in the previous games (at the cost of campaign length being shorter overall), it introduces a corruption mechanic which affects the story and ending you can get. You also get chaos for mp.
Retribution:
This one's a bit of a mixed bag. Campaign is the conclusion of the story started with dow1's first campaign, with recurrent characters from all previous games (or even just mentions, apparently Macha is a slut who walks around naked in her craftworld because you can steal all her clothes in this one).
Despite that it's also the weakest campaign because they tried to have a smorgasbord approach where you could pick any faction, but this translates to cookie cutter missions with only slight differences depending on who you're playing as (I recall nids having the most radically different campaign but was still meh) and a gutted levelling and rpg system in comparison to previous dow2 campaigns. Even gear is more Spartan this time around which is a shame.
Still if you've played the previous dow games and campaigns, you'll want to finish this one.
Despite campaign being overall weaker then previous ones (same deal with soulstorm now i think about it) there's still some fun to be had, Retribution adds in imperial guard for mp and introduces last stand game mode which is a decent enough time waster. It must have been popular because it even got released as a stand alone and a bunch of last stand only characters got added as dlc.
Dow 3:
You didn't ask about it but I will tell you this is how you should feel about that game.
Space Marine:
It's good. I concur with Galdred in that you shouldn't expect some masterpiece but it does what it says on the tin: you're a space marine with a fluff appropriate power level soloing tons of orks (and later csm) and saving imperial guardsmen from dying horribly. It's most notable divergence from other games of its ilk is a focus on melee combat and offense, this was made as a direct competitor to cover focused 3rd person action games like gow series.
Decent fun and the mp scene (assuming it still exists) was fun enough I dabbled in it for a while.