In U.S., I think there are a few regional centered. However, the best (respectable) accents would probably be from the New England states most likely, especially from Boston areas. New York accent and the Texas accents were considered a bit funny and NOT very proper, but they are OK because the people from both placed are probably very wealthy or very tough or both, so you will NOT want to mess with them!!! 
Due to the Hollywood movies and TV programs, California English is also very popular, and the Valley accent (NOT the typical California accent, but just from one area) is considered both wealthy and totally airheaded (very low intelligence like Paris Hilton)!!! Then, there is the Southern accents, and it means different things to different people. To some, it means gentlemenly and ladylike and warm. To the others (especially to the Northerners) ... well... it means it's a bit back countried. Most Americans in the major cities try to speak something more neutral like a mixture of New England and California accents, unless they want to give a certain impression to others.
That common American English is typically called "Mid Atlantic" but reality is it is spoken from Western Pennsylvania all the way West into California (through central Ohio, Illinois, Nebraska, Iowa, etc) more or less. This is how people on TV typically speak as well. The North Eastern accents, from Philadelphia to Maine are not standard. The accents from Southern Virginia (Northern VIriginia near DC commonly speaks Standard Mid-Atlantic) all the way to Northern Florida and West to Central Texas can be described as "Southern" but people in places like Tennesee do not actually sound like people in Eastern Texas, there are different dialects of "Southern".
The West coast was the last area to be settled so it got a good mix and is fairly standard, there is that "Valley" accent but that is a very small minority of people, most people in California do not speak like that.
As was mentioned due to televsion and radio being standardized (or owned) at a national level and the fact AMericans move around more than ever...most people in major cities speak "Mid-Atlantic Standard"...for instance in Houston, Texas, in Washignton DC, in San Diego, in Chicago most people sound similar, there is some hint of regional dialect and the further you go outside the metro commutting area into smaller towns the more dialect you will here, but for the most part it is not anywhere near as strong as in the UK.
Mainly because Americans have historically been pretty mobile and it is a young country. We did not have hundreds of years of stagnant fiefdom where people in relative isolation from each others like in England to form strong dialects.
America was formed in 1776 (officially) and by the early 1900's we had radio, by the mid-1900 we had television and until after WWII most of the Western part of the nation but for Southern California was sparsly populated, so less than 200 years to diverge significantly. New England and the South East were settled first so they had longer, but for the most part we can understand each other fine...there are some places I've been in Kentcky and Texas where I had to ask people to repeat themselves mainly because they had a strong accent and spoke too fast, but these were very small towns.
http://en.wikipedia....tlantic_English
Edited by LongMa, 14 March 2008 - 12:16 PM.
