I'd say Platinum (that's the first 3 right?). You can't beat 3 games for $10. If you're big on singleplayer, it really is like buying 3 games. If you're into multiplayer, it's like one game with tons of bonus content. Either way, you win!
No game is as popular as Starcraft, and few are as popular as WC3. Regardless, you will find plenty of people to play with online. I actually prefer the DoW series because I like how resources are dealt with (a lot like Total Annihilation, you build power-generators anywhere and resource-collectors on strategic points).
Dawn of War + Winter Assault have awesome singleplayer campaigns, but I'm big on storylines. In fact, I wish Relic had gone back and did more on the story that was finished in Dawn of War (which had an ending that could easily lead to a storyline sequel). Dark Crusade's singleplayer was a lot of fun too, it wasn't storyline driven but there was certainly some backstory lingering there.
If you're looking for a good RTS, I highly recommend the entire DoW series. The races and units added in Soulstorm were a little disappointing, so don't feel pressured to buy it until it's in the discount bin (unless you're into the game so much that you simply must have everything like me). Dark Crusade was a fantastic addition to the series (woot Tau) and really should not be passed up for the extra $10.