Yes, generally speaking it would make sense that with the exception of some other countries, for instance, England and Scotland to name a few, English spoken in the US is better than other countries where English is not the primary language. Although the primary language in America is English, Americans are not necessarily strong in speaking what is considered proper English. So if the question is just based on the English language in general, then it could be argued that Americans speak it better than other countries, again, where English is not the primary language. However, if the question is based on proper English, that might be an entirely different debate.
Americans are better at speaking the English language. This is because most of the changes in the English language come from this country. We set the tone on what is acceptable and what is not, and America is also the place where most slang words and abbreviations in the language come from.
There a different forms and dialects to the English language. To compare the United States English toward Cuba's English, than yes the Americans speak better English. But compare America's English to Australia or Scotland? No. Americans English can be really horrid filled with mispronunciations, grammar errors and the likes. Sorry.
I find English spoken with an American accent very hard to understand. I have attempted to speak with Americans from different parts of the USA and some of the accents are not even possible for me to understand. American English is non a pure form of English. The American accent makes them sound stupid.
Textbook English allows only one correct English with proper grammar, proper spelling, and proper pronunciation. This is what other countries strive to learn when they decide to learn English. Americans can already converse with other Americans and do not strive for any level of correct English besides what works in their every day thereby making American English less perfect than English spoken in other countries.
There is no universal standard of English. There are instead many versions of the English language across the world (e.g. American, British, Australian, etc.), each with their own accents and dialects. Therefore it is impossible to say Americans are 'better' or 'worse' at speaking English than other countries; they are simply different.