So Shrine of Storms seems to me to be the most beneficial world in the game. The skeletons are probably the toughest standard enemies in the game (outside of, I dunno, giant goblins), but they'll teach you to time your dodges and attacks, which is the crux of DS' combat, and they offer TONS of souls compared to standard enemies in any other world. You'll probably end up farming souls just by accident as you play through 4-1 and 4-2, without trying specifically to do so. The bosses are pretty easy, comparatively, for a melee player character* and the environs are not too dark or twisty or punishingly trap-filled. (Though Storm King is the first Archdemon I end up beating during each playthrough. All in all, probably the most straightforward and helpful of all the levels in the game.
Now, 'most helpful' doesn't mean best, which I'd say is probably Boletaria due to the excellent design and intricacy of the levels as well as the impression that it imparts upon one's first playthrough, or perhaps Stonefang thanks to strong atmospherics and that same great level design, but Shrine of Storms is definitely up there. Who am I kidding, I love them all - even the Valley of Defilement. Well, kinda.
Been playing this game a lot lately. It's superb, probably one of the best ever. I can pick it up after months of not-playing and the duel-ish nature of the timing of combat just feels perfect and natural, and I feel right at home, like I've not missed a beat. And then I fall off a cliff and lose 30k souls and realize I probably deserved it for not being careful enough. Demon's Souls = <3
*note that most of the bosses in the game, in all worlds, are made significantly easier by focusing on magic or archery, with the Adjudicator being a funny exception. He's quite cheesy for a decently leveled melee PC.