It depends on what will be in your tank and how large/small the tank will be. I personally would say (but it's just my opinion) that generally for tanks 10 - 55 gallons a hang-on-the back power filter would be good (for example AquaClear, WhisperTetra, or Bio-Wheel); and for tanks 75 gallons and up external canister filters would be good (for example Eheim or Rena Filstar XP). But different people would do it differently - some would use h.o.b. power filters on larger tanks or canister filters on smaller tanks. It depends on what you like, and as I have said, on what is in your tank. I have no experience with internal filters, so cannot comment on that. There are also sponge filters (internal) that are best for fry tanks. These suggestions are for standard tanks, without plants, but with just fish and regular gravel and decor. In heavily planted tanks (with special substrates) plants help to filter the water a lot, so in such a tank a powerful filter that you'd normally use without plants in the tank wouldn't be that necessary. As you can see, there are a lot of issues to consider when choosing the "right" filter - what's right for me, may not be right for you.
What is the tank size and fish kinds (as well as how many fish) that you're considering? A heavy load of fish would require a more powerful filter. But it's never good to overstock anyway.
Zebra danios are VERY fast swimmers and a 5 gallon tank is absolutely a "no-no" for them! A dwarf gourami is also too large for a 5 gallon tank ... you don't want it to live in such a small tank, do you? Of course you don't ... so ... neons could work, but just a few. And that's it. Unless you get a bigger tank. With a 5 gallon, I wouldn't get more than 4 neons. You could get a betta, considering in what conditions bettas live at fish stores (in a cup of water or so ... horrible!). But taking into consideration the size of a betta, I would NOT personally put it in a 5 gallon tank. I'd put it in at least a 20 gallon (10 gallon at worst). Please, always remember fish need space too, and if a fish is large, it will suffer in a tiny tank.