What grain broad head would be most successfull with a bow set on 45 pound draw back?

We need more info than that to determine whichweight is right for you.

1)Arrow make?
2)Arrow length?
3)Arrow model?
4)Draw length?
5)Desired FOC
6)What are you hunting?

I shoot 45 pounds right now and I use 100 grain broadheads. Shoots 280 fps too, so I am content

a 45 lb bow shooting 280? holy cow that seems awful fast. Anyways when using a lower poundage bow i would stay away from any mechanicals, instead look for a good cut on contact head, the magnus stinger would be a good one to look at...proven design and not that expensive. you might want to consider a slightly smaller head too, like an 85 grain, but as long as the blades are nice and sharp a 100 grain head should be ok.

As already mentioned, there is more that goes into selecting a broad-head weight than a simple "number". The broad head weight must be matched to the shaft you are using ... which considers both draw weight, draw length and type of cam action if you are using a compound bow. I would recommend that you match to handle either 100 gr. or 125 gr. broad-heads.