Well, even in Spain there're different accents. For example, people from Sevilla, Cádiz or Murcia are difficult to understand sometimes for people from other parts of the country.
CCElias have explained some differences, and I have another too. In Spain we have different sounds for 'z'/'c' and 's' and they don't. For example, the word 'cereza'. We pronounce it as /θereθa/, but they say /seresa/.
Another very common difference is the 'Spanglish'. Specially in Mexico, there're a lot of words adapted directly from English.
And another problem. Some words have different meanings, it depends of the country. For example, 'Concha' has for CCElias and for me a completely different meaning.
But well, there's a no a 'best Spanish'. In each country usually says theirs is better than the others, obviously. My advice is you should learn as accurate as possible for your destiny, but if you learn Spanish, in general you will be able to understand everyone. I knew two Italian girls who learned Spanish in Italy with a teacher from Venezuela. The mix was so curious!
Finally, I recommend this song for you. They explain the differences very well!
https://youtu.be/eyGFz-zIjHE