I have to agree with nittanytbone, the only PRC i'm aware of that is better then basic druid is Planar Shepard. It's powerful if you don't abuse it's abilities, it's retardedly broken if you do abuse them.
Otherwise, for pure power and flexiblity you want druid 20. If you want to focus on one specific aspect of a druid, and are willing to give up some power, then there are a few OK PRCs.
If you want to focus on wildshape and are willing to give up caster levels and animal companion, Natures Warrior and Warshaper are ok.
Or you can drop wildshape and/or AC and get any interesting divine casting PRC that doesnt require turning.
But ultimately, I think that any PRC you go for as a druid gives up more then you get. So not worth it IMO, but depends on the player and character really.
See the problem is that base druid is so good, that unless the PRC gives up a fair amount, it can't get much in the way of benefits without becoming overpowered. Since you said you wanted to avoid overpowered PRCs, you either have to give up some stuff, which is usually some combination of caster levels, wildshape and animal companion, or just stay basic druid.