The best one is probably one you can build yourself with a kit. It's also cheaper to do that. It also depends on how often you want to use this e-bike. If it's something just for the summer, you might want to just get an e-bike that has SLA (lead acid) batteries. If it's something you think you'll want to use for years to come, then you might want to invest in an e-bike with better batteries like ones with lifepo4, lithium-ion, or NIMH.
You can find this bike for $850 but I think it uses SLA batteries. SLA batteries is the worst battery for an e-bike. You get short range and they are just kind of awful for an e-bike. I don't recommend them at all unless it's just for the summer.
There also the the Izip Zuma. There's the Ezee bikes. Look up "ezee electric bike" on google. Those are the popular brands. I don't know how well the hybikes.com bike does on hills. If you want to build your own, the ezee motor does really well on hills.
It's $1200 but it comes with a lithium battery which is really want you want if you want long range and longevity. You could probably get the kit without the lithium battery if you wanted to use SLA instead.
I have some experience with the Lightfoot Cycle products. I bought an electric recumbent from them for my son several years ago and it is still the most comfortable electric I have ever ridden.
Lightfoot is now making a bike designed solely for electric. The Smoothie can accommodate a Bionx or Eco Speed electric system. As soon as I can justify the expense I am going to be ordering the Smoothie with the Eco Speed system. With 36 volts driving the back 8 speed cassette and the recumbent comfort I doubt if there is anything on the market that can compare in both comfort and performance.
I am very new to electric bikes and many here have more information than I do. That said, if you are looking for an electric bike that is ready to ride (no DIY required) and that you can buy at a regular bike shop, check out the latest Trek electric bikes. http://www.trekbikes.com/us/en/bikes/electric_bikes/
They seemed to be designed to be exactly what you are looking for.
If you have a mountain bike that you like, adding a kit is pretty easy. I did not so I purchase a value price mountain bike and converted it. I would not use anything less then 500w/36v & lifepo4 battery. As I want to use it more like a low end scooter then a pedal assist.
The total cost including racks, lights,mud guards to me was $1200. If I could get that from a bike shop for less then I would go for it.
Soma Buena Vista mixte single speed commuter, Aster 27 speed folder, Mercier Kilo TT Gold Digger fixie.
It really depends on what you want the bike for.
The first decision you should make it whether you want a pedal assist or throttle control. If this is a bike for exercise with a little extra power for towing a carrier, then throttle assist is probably all you need. If you want something to actually get you around without pedaling then you'd want a throttle bike. If it's for mixed use then you might want both options.
Many on here are advocates of building your own, but I think that has a lot to do with how handy you are and how much effort you want to put into the job. I've personally found the custom built bikes to have the wrong proportions once you add the weight of the motor and battery so I'm more drawn to the purpose built e-bikes. I've ridden a few but haven't yet found "the best" for my needs.
the best are Tadial Force/recentlyE+/ and Optibike.
there are no secret about it.
There are both USA-made.
I owe Tf and can tesdtify to incredible durability of its motor/contrioller.
Original NMH batt was cause of problems but after replacing batt with LiFePh my TF is rock solid. and very efficient.