When Sales go Wonky

I don't actually own any of them, I got a free copy of Chaos Rising from Kinguin and gave it away.

That depends on what you want to play. The campaigns in DoW 1 & its expansion, winter assault, are great. Dark Crusade has a more non-linear campaign, and has more replay value due to it, though it's also very easy abuse the system. Soulstorm is buggier, and some people don't like the additions, but I personally enjoyed the new factions. if you just want to play skirmish go with either Dark Crusade or Soulstorm.

As for DoW 2, Retribution has a shorter campaign but better skirmish. The original game and Chaos Rising offer different campaigns. Personally, I found these campaigns to be dull as dishwater, but some people liked them.

As for retribution DLC, they can be split into two categories:
-Last stand Stuff
-Cosmetic stuff

The last stand stuff gives you more to play with if you want to play "last stand", which is a multiplayer mode where you and two friends try to beat back hordes of enemies. One of the DLCs gives you a new commander that's not just a model from the main game, so it might be worth buying if you really want to play last stand, the equipment on the other hand is really not needed. The cosmetic DLC looks nice, but it won't actually impact your game. If you love a specific 40k faction, and there's a DLC for it, it might be worth buying, but otherwise you can easily skip it.

I'm a big fan of tabletop Warhammer 40k. I'm a big fan of the original DoW and had a lot of playtime in it even before I had Steam. It's getting a bit dated but it's still a solid RTS and worthwhile if you can get it cheap. Dark Crusade was probably the weakest expansion and Soulstorm likely the best.

I got DoW II in a humble bundle but haven't played it enough yet to offer serious opinions. It comes fairly highly recommended, but it is more of a squad tactics game on a smaller scale than the first. A lot of the DLC looks cosmetic and/or junk though.