Best I don't know about the best, but the cheapest is the 3s Parkzone battery that I put into a TC6B radio...
The lipo was headed for the trash bin until I did the mod. Works great and could be done with the 9x radio just as easily.
I only went through one set of 8 AA batteries before I decided to do this mod... I hate throwing money down the drain.
I use the Turnigy 2650 3s 1C. Love it. With HK batteries, they have a separate warranty protocol. When you get the battery, you register the number on the barcode. If there's an issue e.g. dead cell, all you have to do is take a photo showing the battery hooked up to a checker and the numbers on the screen. HK will send you a new battery straight away.
I love the Turnigy 2650mAh 3S 1C - but unfortunately I can't use it in my Spektrum transmitter. I would have loved that! I have used this battery in my old 72MHz radio, and only charged it once in like 4 months!
I have the 2650 in my Dx7 as well - it has been working perfect since I bought it - sometimes I charge the LIPO above 12V - sometimes I stop charge at 12 V as I did not install any Diodes to drop the voltage.
Fits perfectly in the 9X all cells arrived balance on both my battery an the battery of my friend. I dont get why you dont want to ship them overseas, all my friends buy the batteries from hongkong (hobbyking) not a single battery had a dead cell and all the batteries where perfectly balanced. I am talking about 20+ lipo batteries (rhino's, flightmax, nano tech, zippy and turnigy)