I've played them all. First Dark Souls, then Demon's Souls and finally Dark Souls 2.
I can't choose a favourite one. They're all amazing. Perhaps I'll go with Dark Souls 1 on this one.
Because it was the first Souls game I've played (Demon's isn't on 360), and it struck me how amazing the game is. Also, Dark has the best bosses out of the bunch, and the fact that the map is entirely connected instead of a hub makes it amazing too.
What's the problem with DS1? The story. The story is the worst one, considering the lore is amazing. The ending is just an insult to the player, despite the implications of it.
So then there's Demon's and Dark Souls 2. The thing is, I think the atmosphere, gameplay and everything else is better on Dark Souls 2. Didn't pick it because the bosses are a breeze in comparison. But I think that after replaying it on NG+ it may become my favourite one. Then there's Demon's Souls.
Demon's Souls has some charm on it. Something that isn't present on Dark Souls games. Maybe because it was more about the Demons and less about the Souls, or...I don't know (some boss battles were horrible though). Despite what I said about the map, I really loved the hub as well (That's why Dark Souls 2 is ace, it combines both things of the maps of Demon's/Dark). And the atmosphere was heavy. Also, it had more difficult spikes than Dark Souls 2.
So really, I can't choose. I'm going with Dark Souls 1 this time, but maybe in the future when I replay Dark Souls 2 or Demon's Souls they'll get the crown again.