My filter set up looks like this...

Lee foundation kit w/ front mount ring
Wide angle adapter for wide lens
Standard adapter for telephoto lens
B+W 105mm CPL - w/ 2 slots on the holder there's no vignetting after 20ish mm
Lee .6 hard GND
Lee .9 hard GND
Lee .6 soft GND
Lee .9 soft GND
All GND filters are 4x6"
Lee Big Stopper

I've ordered the Hitech Prostop 6 and 10 stop ND filters. I'll probably sell the Big Stopper. I'm also considering adding a Singh Ray 3 stop reverse GND filter. I'm a little hesitant due to color cast problems I've had with them in the past. This filter can replace the Lee .9 hard for most situations.

So as far as recommendations I recommend the Lee GND filters over the other brands. It can be frustrating to buy though because they're frequently out of stock. They also don't offer a reverse GND so if you want that you have to go with Singh Ray or Hitech. Can't really make a solid ND recommendation, but I'm afraid I'll break the Big Stopper so I'm giving the Hitech ones a try.