Almost every OTA you find on Kayak, Skyscanner, etc is legit, in terms of: they do actually issue flight tickets. However most of them charge high fees for extra services (bags, insurance, seats) that are not trivial to avoid. Plus their changing and cancelation fees are extremely high.
If you wont need any extra services or dont plan to make any changes to the booking you can just book at the cheapest OTA.
If you want more service, book directly with the airlines.