That depends on your budget and tastes.
Fredhead, Pedalchopper.com billet head, Jakes diamond head, VeniceMotorBikes new swoopy Slant head, too many to choose from.
I run a FredHead on My Pig, and a Pedalchopper billet head (for clearance) on my Occ 5spd build.
I love them both.
If you're on a tight budget, the Puch 70cc hi comp head is every bit as good as any billet head for improving cooling and compression and can be had for arund $40 or so off fleabay or treatland.com