It depends on your needs
I have used the stock, Merlin and Shibby firmwares. Merlin's seemed to provide the best stability and wireless throughput. However, I have stuck with Shibby. It has been resonably stable and it offers, in my opinion, the best parental controls through its "access restrictions" set-up. When it comes to parental controls, I had endless problems with the stock firmware; it looks good but, in effect, it is not reliable and not flexible enough. Merlin has not tinkered with the parental controls.
So, I so suppose it depends on your needs and set-up. I am willing to sacrifice a bit of wireless performance in order to have better parental controls. If and when the kids get more responsible or I need the signal to travel further then I would move back to Merlin.