As you can see, eBikes have lots of options. If you think about it, eBikes have all of the options to consider that standard "acoustic" bikes have plus all of the options related to the battery, motor and computer. As with anything, the quality of each of the parts will affect the performance of the bike.
The components of the bike are going to determine how comfortable it is to ride and how easy it is to maintain, and how efficiently it moves. The electric components will determine how naturally the motor responds to your pedaling and how much range you will get.
Consider when you buy a bike, if something on that bike breaks, can you (or a bike shop) get replacement parts for it? This is often a difference between name brand, reputable companies, and cheaper bikes. If a part breaks and you can't easily replace it, you will have to do a DIY conversion or buy a new bike.
If you have the opportunity, test ride a bike from a dealer, rent a bike, or try a friend's bike to see what it is like. Look for online reviews of the bikes you are interested in.
Everybody has different needs and priorities, but for most people in most circumstances, you should consider the following as a starting point when purchasing an eBike:
Purchase a bike from a name-brand dealer like Trek, Giant, Gazelle, Specialized, for the easiest experience and support. Alternatively, buy from a well-known online manufacturer with a proven track record, like RAD Power, Aventon, Electric Bike Company, Charge, Priority. If you are good at Do It Yourself, you can build your own bike for considerably less than a name brand bike (but it will require some research and time, and will probably not have as clean a look as a purchased eBike).
Purchase a Class 1 (or Class 2) eBike. In general the number of trade-offs with a Class 3 eBike (higher cost, shorter battery life, less trail compatibility) are not worth it.
Mid-drive motors outperform hub-drive motors. They feel more natural, integrate with the bike's gears, are better at hills, are more balanced, and more efficient
Most people should look at commuter/urban style frames with larger-diameter, medium-wide tires, unless you are looking for a more specific use-case (beach cruising, mountain biking, or utility)
Bikes that combine readings from speed, cadence, and torque sensors provide a more natural, responsive feeling ride than single sensor bikes. If you can't afford a bike with multiple sensors, torque sensors are better than cadence sensors at producing more natural rides.
Look for bikes with hydraulic disc brakes
Front suspension will make for a smoother ride
Higher end bikes will contain higher quality components. Better frames, saddles, gears, shifters, brakes, wheels, tires, suspension, motors, and batteries. Batteries will be integrated into the frames, for a much cleaner look. Bike cabling will be routed inside of the bike's frame, for a much cleaner look. All of these dozens and dozens of components will contribute to a nicer riding experience.
If you can afford it, strongly consider a bike with a Belt Driven Internal Gear Hub. Virtually silent and maintenance free, change gears while pedaling or stopped, more gears
Electric Bike Reviews - This site provide really good reviews of popular eBikes. Most reviews provide videos and specifications often not found on the manufacturer's website.
Propel Bikes YouTube Channel - This site has videos with useful information about eBikes (like how to size your bike, or make it more comfortable)
People For Bikes - eBikes - This site has information about laws and regulations relating to eBikes
Komoot - With this site you can plan your own bike route, and the routes can be synced to many bike computers
Strava - Another great route planning and bike tracking website with a strong social component
eBike Maintenance - A Juiced Bikes article with really good maintenance advice