North of Wellington isn't worth it if you're after proper singletrack. I pretty much only ride there when the normal trails are too muddy. Map
tells you all of the directional trails. The loop taking in the blue trails is the fairly standard loop that most people will do.
If you head clockwise, the north east corner of the blue loop is the fun flowing bit. If you head anti-clockwise, the north west corner of the blue loop is the funner flowing direction. Blair Witch in the south west corner is downhill heading anti-clockwise.
(not to say that the fun flowing bits are all downhill flow, there's still climbing, but I personally think certain sections flow better in certain directions).
The Commonwealth Games / Competition track in the West is all up a bit meh. Loose gravel over hardpack, narrow and lots of braking bumps.
If you're not confident with your technical skills, you'll probably find Follow Me, Aneurysm and Granite Link a bit too much. They're tricky and technical trails, but very fun when you can do them well. If you want to give them a crack, I'd try them in that order. Watch out for the big boulder drop at the top of Follow Me (there's a B line). Most of the gap jumps and the big log wall have been removed from Aneurysm, so it's faiiiirly safe, but there's still plenty of endo potential on the little jumps.