The Bio-wheels in the Hex5 kits are good or if you are not planning on getting an aquarium complete with filter the size of the tank is going to make a difference but the EHEIM Liberty HOB filter is good with a small variation of putting a bit of filter material in the outlet to slow it down a bit. It keeps the water really clear.
My favorite filters for small tanks are sponge filters. These usually have to be ordered online as most stores don't carry them. They create little current and I have observed Bettas pecking microorganisms of the sponge--they must love the live food. But, a regular hob works just fine also as long as the current isn't too too strong.
Any HOB filter will do as long as you block the water outlet with some filter floss or filter sponge media. The water still flows through but there is no current. I just jam the filter floss between the water outlet and edge of tank hood.
The AquaClear HOB filters have an adjustable water flow so you can make it a little less strong. I find if I keep the water level fairly high and the water doesn't have far to fall, it is usually fine for my fish.