Bosch is tried and tested.
Some reports of drive side bearings failing due to crud ingress, but I wouldn't worry too much about that unless you plan on doing axle deep mud plugging.
The Yamaha motor appears reliable and rides similarly, although there are a lot fewer in service.
You might come across bike with Shimano's fairly new E8000 (I think) mountain ebike motor.
I expect that will also ride nicely, but there are some reports of it being noisy and overheating on tough climbs.
Shimano coughed that job to a limited extent saying the cooked ones were prototypes - make of that what you will.
Your budget will get you a full sus or hardtail.
That's your choice, but think of it as a push bike with the cost of the motor and battery deducted.
Leaves about £1,500 for the bike bits, which would get you a very good spec hardtail, but a relatively low-spec full sus.