If you're going to use the Flamberge go 18/21. If you use any less agi you'll find it a bit slow and in turn find it hard to swing back at agi whores etc without getting spammed. With 30/9 you’ll definitely get spammed easily. I wouldn't recommend going any lower than 18 agi. But 18/21 is superior. You still do nice damage and as Malaclypse said, WPF also increases damage from the speed bonus you get. Also with footwork, the higher agi will generally have the better footwork (if it’s an experienced player). I've always found 6 ath to be the sweet spot. Decent speed and maneuverability, I find it quite a step up from 5 ath.
Lvl 30 18/21 build with Flamberge
Lvl 31 18/21 build with Flamberge
I’ve used these builds in the past and they shred. I am actually going back to the Flamberge after I get rid of my Masterworked Heavy Throwing Axes (damn you chadz) and will use the 18/21 build again. It pwns and it’s fun. If you’re going to use any other 2h I recommend 21/18 since you have 7ps and 6 ath. A nice balance and it’s also superior to 24/15 and all the others in my opinion.
Lvl 30 21/18 2h build
lvl 31 21/18 2h build
If you're going to switch between the Flamberge and other 2h weapons, go with the build that is suited to which weapon you use the most. 18/21 if you're going to use the Flamberge the most or 21/18 if you're going to use it less than any other 2hers you're thinking of. Though in my opinion the Flamberge is the most fun 2her and shreds all others at 18/21. Decent speed, great length and one hard hitting son of a bitch. It looks great and performs even better when Masterworked. What a gem.