F11.
As somewhat overpowered the Knife is (due to extreme versatility and 1-hit kill ability), I reckon that it is practically fine the way it is. Being able to shoot a knife away makes it useless, plus that gives rise to spraying and making autos more powerful than needed. Plus, it takes too long to switch to another weapon after you've thrown your knife and then shooting the knife to make it fly farther and faster.
Besides, you can kill a knifer with his own knife. Take a look:
![](http://img524.imageshack.us/img524/329/soldatknifeandgrenadekijc9.gif)
^This is a very famous gif around here, an epic knife self-kill via grenade boosting caught in older Soldat versions, not sure who did this.