Which aquarium air pump is best for my fish tank?

Answering your question depends on knowing the tank's size, the number of fish you wish to keep, and the type of fish. I have kept a 10-gallon tank for bettas and tropical fish. I have tried both the hang-on-back and the submersible air pumps. The hang-on-back air pumps look nice but are not as powerful as the submersible ones. I also found them less durable. The submersible ones are hard to clean and often require replacement before the tank is half full. A good, basic air pump for 1 or 2 fish tanks is the Current USA B100. The pump is capable of pumping up to 100 mph. This is more than enough for the small tanks, and the pump is small; it does not require a lot of space. It is not very quiet, but considering how small it is, I don't mind the noise. Another good pump is the Aquatec 300. This pump is also capable of pumping up to 100 mph. It is quieter. I used this one in my 10-gallon tank and it worked very well.

I have not used a stand-alone air pump in a long time. I have one I never used. I bought it for making a micro tank for my beta when the main filter broke but just ended up getting a new tank that came with a new filter.