Friction Drive Conversion: This type of system doesn’t really work with anything other than slick tires. There’s none in this list for that reason but you can still get a sense of what they are. If you’ve got a mountain bike you use for commuting duties you could change tires and make one of these systems work. A roller pushes against the tire on the wheel and when the roller turns the wheel turns. This strategy isn't all that efficient but it's simple. There's very little challenge with making it work but at the end of the day, it doesn't work all that well.
Mid-Drive Conversion: The best electric bikes tend to be mid-drive and the same is true of conversion kits. The weight sits low in the frame and the power gets applied to the crank for a more natural sensation. The only downside is pricing and packaging. Different standards make it challenging to figure out exactly what you need and there's more work involved in adding the parts.
Electric Bike Wheel Conversion: Swapping either a front or rear wheel for an electrified version is a good balance. The conversion process is very simple and depending on how the battery mounts the weight distribution can be quite good. Powering the wheel does change the way the power delivery feels and making the front wheel heavy can affect the handling of the bike. If mid-drive seems overwhelming this is an excellent option.