It also depends on how hard you're bouldering? and if you were looking for a bit of scenery as well as climbing??
A good pick, with both hard and easier bouldering - and FREAKING stunning scenery, would have to be Taipan wall, and Trackside boulders. It's pretty easy to find trackside and Taipan - you camp at the same place - Staypleton Campground at the Northern end of the gramps. Park at the Staypleton Picnic Ground and head up "Flat Rock" following the painted markers on the rock. At the top you take the path into the amputheatre off to the left and go down into the amputheatre. Taipan wall is the obvious 200m high overhung orange wall in the middle.
About half way across the amputheatre you'll find a rough track splits left up through some large boulders - This area is "Trackside boulders" and the "Amazing" boulder is the really big round one! Heaps of awesome problems round this and the other scattered boulders. From here you head up past some very highball V3's, and follow the track directly up the steepish hill till you get to the base of Taipan wall.
From here head right along the face till you come to a long sandy area with TONS of chalk. Can't miss this. Great traverse and plenty of up eliminates (easier area than Trackside if you want it to be). This area stays in the shade till about 2pm coz Taipan is so overhumg. So if it's hot, and late December can go that way - then it's a great spot to start and end the day.
A walk to the top of Mt Staypleton is worth the effort too - it follows the same path to Trackside and then goes off to the right. More info from the parks website I think.