Is There an Official Weather Briefing?

I have been using 1800WXBRIEF.com and Aviation Weather Center for years since they don't require a paid subscription. But according to the CFIs at the school I just started flying with, these are not considered legal weather briefings. 

Answer: The question asked begs another one: Legal to whom? 

FAA regulations, notably FAR 91.103, require pilots to obtain weather reports and forecasts. However, according to an FAA spokesperson, "the FAA does not prefer one weather source over another, nor do we define a 'legal weather briefing.' It is up to the pilot in command (PIC) to use a weather source that best suits their needs and allows them to meet the preflight planning requirements."

That being said, there are some CFIs and flight schools that advocate paid subscriptions, such as ForeFlight, and free discreet login services, such as 1800WXBRIEF, because in addition to providing information, they also allow the pilot to file a flight plan. They also require an account, which means it's easier to prove the pilot obtained a weather briefing prior to the flight because there will be a record of the login.

The latter is often one of the first things the National Transportation Safety Board checks when it investigates an accident or incident.

At the very least, a pilot should check TAFs, METARs, winds aloft, and NOTAMs prior to a flight. It is distressing how many pilots and pilots in training believe that listening to the ATIS/ASOS/AWOS at the airport or along their route constitutes a weather briefing. They don't. 

Nor does looking out the window at the FBO. Any more than “pretty good” is a PIREP.