http://www.channelfireball.com/articles/the-top-5-legacy-decks/

It has the best threats of all the Delver lists. Deathrite Shaman is still a hell of a card, and Young Pyromancer is one of the best creatures in Legacy. Young Pyromancer has always been an underrated card. For a long period of time, I felt like it was the best creature in Legacy and while it’s not as powerful as it was when Treasure Cruise was legal, it’s still quite good.
In addition to having powerful threats, Grixis Delver also has some of the best disruption since it gets access to Cabal Therapy alongside Young Pyromancer to really tear apart the opponent’s hand, and it can also play grindy cards like Painful Truths in the sideboard to switch to a less tempo game plan and to a more grindy game plan post-board against slower decks.
I think Grixis Delver is also favored against Miracles, something that other Delver lists can’t boast. Having a good Miracles matchup is pretty key to being successful in Legacy.