What kind of filter should I buy for my aquarium?

As per my opinion, If you have a freshwater aquarium, tap water is the best kind of water to use.
Tap water has the right level of pH, it's usually not too hard and any harmful microbes have been killed at the treatment facility.
The only problem with tap water is the presence of chemicals like chlorine and fluoride that can harm or even kill fish. But you can easily solve this using an aquarium water conditioner that dechlorinates the water. If you want to read more information then click on the following site
https://cleanersofterwater.com/water-treatment-101/

I think external canister filters are a great choice for maintaining great water quality and the better choice for many reasons.
1) they take up no room in the aquarium except for the inlet/outlet pipes.
2) most have both biological and mechanical filter media in one unit, and you can swap out/add in new media as required.
3) very easy to remove and clean without disturbing or stressing the fish
4) more equipment can be added inline (such as heaters, uv sterilisers and booster filters)
5) the units can live in the cabinet underneath the aquarium out of sight.
6) generally very quiet operation

The reason a lot of people are put off by external filters is the possibility of a leak. The seals in cheaper units do wear out occasionally and a leak will result in water siphoning out of the tank.
Some people try to limit the amount of water that can siphon out should a leak happen, by placing the strainer outlet just below water level. This is not recommended as the strainer needs to be placed as low to the gravel as possible to suck up heavier debris.
A better solution is to drill a small hole in the outlet pipe just below the water level. But place the strainer at the bottom of the tank as normal. If a leak occurs the water level will lower to the hole before air gets sucked in and stops the siphoning/pumping. You may still get a patch of wet floor and a ruined cabinet but it's better than thousands of pounds worth of water damage.

In my opinion, power filters are the best. They are easy to maintain and are also quite economical. The most common style of power filter hangs on the back of your tank. A siphon will pull water from the tank, eliminating any bacteria and returning filtered water back into the aquarium.