By all accounts the Bosch, though there have been some good reports of the 36 volt Panasonic system.
Although I've been a long time supporter of the Panasonic crank system, since they've been playing catch-up the differences have been diminishing, losing some of the better Panasonic characteristics. At the same time the small separate motor sprocket has caused some problems in transmitting greater power, so I think the chainwheel drive of all the others is superior.
That aside, the Panasonic has been a little more reliable than the Bosch, but the odd Bosch fault has been repairable, internally the Panasonic is not, a fault means a new unit at circa £500 once out of warranty.
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