Speech is the primary medium of language.  It uses sound, produced by the vocal organs and perceived by the hearing system. Phonetics is the study of speech.  more A dialect is a variety of a language shared by a group of speakers, defined regionally or socially. An accent is the way a given dialect is pronounced.  more Many people have preferences as to how English "should" sound.  But from a phonetic point of view, no speech sound is intrinsically "better" than any other.

Accents differ mainly in their vowels.  The vowels of British English are presented here.

Nonetheless, accents have different degrees of prestige, and some accents become accepted as "national standards", favoured in the media and used as models for foreign learners.

The "national standard" of the USA is General American.  This is by far the most commonly heard accent in American films and TV.  Originating in the Midwest, General American (or something like it) is spoken by a large section of the US population, across a range of social backgrounds.

The standard British accent, by contrast, is a minority accent associated with higher socio-economic status. However, the huge social changes of the 20th century, along with natural shifts in pronunciation over time, mean that the traditional "Received Pronunciation" (R.P.) is now out of date.  More on standard BrE, R.P., Estuary English and Cockney here.

My own published research includes this article on vowels in British and American dialects:
Lindsey, Geoff (1990).  “Quantity and quality in British and American vowel systems.”  In S. Ramsaran (ed.), Studies in the pronunciation of English, 106-118.  London: Routledge.