@romeo This is completely disregarding mods, because I didn't use any when I first played DoW, so Soulstorm is by far the weakest. With UA you could say its the best, but I always personally preferred FoK for DC since it was smaller scale so each unit mattered more and had a wider range of application, while UA usually just boils down to who gets the BFGs out first. Which is fun at first, but gets stale for me after about an hour at most.
Soulstorm was weaker for a few reasons which all stemmed from the game being too ambitious.
The campaigns where slow and grindy (and the campaign is the bread and butter of DoW, at least for me) it was often the most optimal thing to sit still and collect resources rather than push for new territory... which is tedious and boring. The increase of factions and significantly larger campaign map made moving around and conquering strongholds in the later stages of the campaign a mind-numbing chore. Especially since almost all the banter and most of the character was taken out of the stronghold battles, which I again pin on over-bloated and ambitious content additions. There were more repeats and filler battles as a direct result also.
Outside of campaign the new air units were terrible, and suffered from pathing issues at launch. Pathing issues and bugs in general were pretty awful in SS, more so than I ever remember in DC, I even had several campaigns were I had to start over due to game crashes (I'm sure I don't need to explain why that is frustrating, especially when I'm already frustrated by the pace of the campaigns). And there were many complaints about Chaos and Eldar being OP among my friends, but I never really bought into the balance too much since I was invested in the PVE content.
DC by comparison did everything SS got wrong, the campaign was fast, but not so fast it felt unfulfilling, with what I feel a perfect mix of unique battle modes and compelling strongholds to keep the campaign from going stale. I still go back to revisit Dark Crusades Campaign, something I have not done for SS in years.
WA is a totally different beast, but I still play it even now because the atmosphere in that game blew me away. I have not come across another game that captured the heart and soul of 40K quite like WA did. The bleak snow and grey Gothic city-scapes were the perfect backdrop for the rivers of blood as waves upon waves of guardsmen charged well armed and entrenched Chaos Space Marines. Perfect portrayal of the guard. The Eldar are sneaky and never give straight answers, all their missions being primarily stealth or hit-and-run based, perfect timing and management of finite resources being key to victory. The Orks were this massive mob of units lead by Gazguk, who was the coolest character ever when I first saw him, and the Chaos were just insane. It all came together to be a masterpiece that is burned into my memory as the best 40K game I ever played, only really DC kind of ecpliced it, because it gave me something totally new and unexpected that I greatly enjoyed.
TL;DR More doesn't equal better, DC and WA were vastly superior games for what they gave to you.