Cristina - asking which are the best condos in Miami Beach is like asking a 5 year old in a candy store what his favorite candy is. Miami Beach's architecture is so diverse and full of choices and your price range will ultimately determine where you end up buying.
I personally love the Continuum which is located at the southernmost tip of SoBe and is a full amenity building with valet, spa, restaurants, beach......and not to mention the unending breath-taking views.
But then you have a lot of popular buildings depending if you want to be on the ocean or bay .
Canyon Ranch is HOT now and the amenities are amazing.....I can sit here all day, but at least I mentioned my favorite 2.
My advice is for you to take a trip to Miami Beach and visit the buildings and see their views and what they have to offer, it will not be an easy choice.