What is the best 48 volt rear hub set up for a 35 mph bike?

Ebikes-ca has what you want. 5304 crystalyte motor. Should do 30 mph easily on 48v. Run it on the 36-72v start immediate controller, 48 amp. Brace yourself, if you think $800 for the motor controller and throttle is high dollar, wait till you see what a battery that can handle that set up costs!

Slas would work of course, but they'll weigh about 60 pounds and be drained in just a few miles. Speed is expensive, and good range with speed is reallly expensive! Some are getting the amps they need out of RC racing lipoly batteries, but most people doing that are real experienced with lipoly and know how to use them without setting the house on fire charging them. Others know where to score warranty returned cells from lithium power tools and build thier own packs. The third option is to build a pack using PSI or Headway high discharge rate lifepo4. That is a bit easier, but for those discharge rates, all the pack builders are having to build thier own bms. The fourth option is the easiest, but still pricey, 4 nicad or nimh packs from ebikes ca. You have to double them up to get the amps, and then double them again to get to 72v. Pretty much all these battery options cost well over $1000.

Drop your expectations to 28-30 mph, and things get a lot cheaper. at 48 v lots of the kit motors will do close to 30 mph. The fast version of the nine continents motor is one of the fastest for less money. A 20 ah lifepo4 pack with lower discharge rate will power the smaller motors. Or just two of the nicad or nimh packs.

If you decide to build a battery, go to the endless sphere, and read the battery section for about a month. More info there about building a battery than you can stand.