As a Spanish teacher in the United States for over 30 years, many students often ask me which Spanish speaking country speaks the "best" Spanish. Well, obviously that question inherently will produce some sort of opinion in its response, not to mention that it is a loaded question as well. Furthermore, since most U.S. textbooks and classroom teachers have emphasized Castilian Spanish from Spain for many years, most people want to believe that Spain has the "best" Spanish. However, I always respond to my students that there is no "best" Spanish, and that if you want to hear good Spanish then you must meet and converse with educated people, and that their country of origin is irrelevant. A university professor from México is going to speak a more educated Spanish than a landscaper from Spain -- and vice versa.
Before the Internet, it was true that most teachers used what they had -- textbooks and other materials printed in Spain. However, ever since the Internet has become ubiquitous in classrooms, teachers finally have the resources to teach and explore other dialects of Spanish from many other countries, not just Spain. For example, there are many great websites and online Spanish teachers from México, Colombia, Argentina, and Spain. Of course, you can find an online teacher from pretty much any Spanish speaking country, but these countries seem to dominate the online teaching platform, and for good reason -- they are the most populous Spanish speaking countries, and therefore have more followers and influence.
So, to get back to the question at hand, I will give my professional opinion as to which dialect of Spanish I teach in my high school classroom and with my online students. Since I teach in Arizona, I definitely incorporate more of the Mexican dialect in my instruction, but I also enjoy teaching all of the differences among the other dialects, primarily from Colombia, Argentina, and Spain because those are the ones I am most familiar with and have the most international presence/influence online. As a Spanish teacher, I try to teach a "standardized" Spanish for my students here in the U.S., one that includes the most common verbs and vocabulary that are heard on the Spanish speaking news, television programs, and movies here in the U.S. and México, the most populous Spanish speaking country in the world, which happens to be our neighbor as well :)
In conclusion, I believe there is no one country that speaks the "best" Spanish, but if you want to hear educated Spanish you need to talk to educated people or watch YouTube videos (there are a plethora of them) from Spanish teachers from many different Spanish speaking countries. As far as accents go, that is obviously a personal opinion. I really like the Argentinian accent because of its singsong intonation due to the influence of the millions of Italians who emigrated to Argentina after WW II. It is estimated that more than 60% of Argentinians have some Italian ancestry in them. However, for my students learning Spanish I like to use videos of online teachers from México and Colombia because I feel they have the most neutral and easy to understand accents. As far as vocabulary goes, I prefer the Mexican dialect since they have the most similar vocabulary to our heritage students. (students whose parents are native Spanish speakers). If I were a teacher in Florida, then I obviously would incorporate the Cuban dialect in my lessons, just as if I were teaching in New York City, then I would be using a lot more Puerto Rican or Dominican vernacular in my classroom.