This is a simple question but can have a very long answer. The only way I can give you suggestions is for you to answer questions.
These questions are IN ADDITION to the normal size fitting and other basic bicycle things.
I suggest that you get a piece of paper and pencil to write down your answers before making any decisions:

1. 1. How are you planning to ride? On the road? Offroad? Gravel trails?
   2. What distances do you expect to cover before recharging? Under 20 miles? 20–30 miles? Over 30 miles?
   3. Is the terrain, flat, hilly, or mixed?
   4. What’s the weather like in your place? Cold or warm? Is that good for riding outdoors?
   5. What is your weight? Will you be carrying or pulling anything or anyone? How much does that weight?
   6. Are you a person who wants to get somewhere as fast as possible with little or no effort on your part
      or are you willing to put some effort in while going at a lower speed?
   7. How fast do you need to go? This will determine the type of motor and power output you need.
   8. Do you prefer a traditional bicycle style or a more modern, urban style?
   9. Are you willing to spend the money for whatever the answers to the above questions reveal?

Alright. let’s forget about brand names since they rarely reveal which bike is best, or best for you. instead, let’s go with some baseline minimums:

For a bike that will be riding only on the road, I recommend a rear hub motor.Offroad should be a center-mount motor.
All others can be either.A rear hub unit for rides under 50 kilometers between charges with a 10 Ah battery will do.
50 to 80 kilometers should be 15 ah. Over 80 kilometers should be 20 Ah or more.
If you, combined with the bike AND cargo, exceed 200 pounds but not over 250 pounds then expect a 50% reduction in range
or add an additional 25% in amperage minimum for every 25 pounds. Here it is important to consider that you should never
intentionally drain the battery to zero- you should not go below 10%- and it is best to max out charging at 90%…
doing this will make the battery last MUCH longer.
For mild/flat terrain a 36V 250W motor will be good. For hilly terrain or heavy loads, you may want to go as high as a 48V 500W or more.
Fair warning- prices go up exponentially after you get past 48V motors.
The less effort you want to put in the more battery you’ll need.