Difficult Spanish accents for native-Spanish speakers...
As a native-English speaker I can confirm that native-English speakers often find it difficult to understand other native-English speakers from other parts of the world both in terms of accent and dialect words.
In the film 'Che', the protagonist, Che Guevara, travels from his native Argentina to Cuba, whereupon it is depicted that the Cubans find his Spanish a little odd, and vice versa. There are even occasions when he has to ask them to explain certain words.
As a non native-Spanish speaker learning Spanish, this is both refreshing yet daunting. On one hand it's good to know that even natives experience difficulty with the language, yet on the other hand I think, 'well, if they find it hard, how shall I ever cope ?'.
What is/are the hardest Spanish accent(s) for native-Spanish speakers to understand ?
6 Answers
Like you said falda, the phenomenon happens even in your country. I have heard people from the UK talking to others from the UK also stating they do not understand any of what that person says.
In Chile, we don't have that problem per se, but there are people who speak weird, most definitely, and these people are not confined to a social status or geographical location in particular.
Then, adding to what txelis stated, there is problem of utilizing slang proper for that region etc...
No problem, it is a matter of little time to start understanding the different accent you might be exposed to. The real problem is slang, which by definition is not the language proper that you are pursuing.
Sí?
One will always have "lexical" problems because the vocabulary differs greatly from region to region. One example is the word "computer" in Spain it's "ordenador" a masculine gender noun. In most of Latin America it's "computadora" a feminine gender noun.
This is just one of thousands of differences from one region to another. However the topic, I think, was "accent" I don't have much trouble with anyone's pronunciation except, in some respects, Cubans and Puerto Ricans. The ones I really have trouble with are the Argentinians--but only when they are speaking real fast.
The English speakers that give me a bad time are the Scots, the Australians, and
people from the US State of Mississippi. I am convinced that they (the Mississippians) are aliens who have beamed down from a UFO. There's NO WAY! 
I forgot one. The gallegos. from western Spain (I don't know how to say "gallego" in English). Julio Iglesias is an example. I have a video of him singing ten well known Mexican standards. I know these songs perfectly, but I still cannot understand Julio!!! His son Enrique, is another matter. He speaks proper Castillian. No problem there.
One time in class, I played a video of Julio speaking to an interviewer. In my class at that time I had several native speakers, some Mexicans, two Spaniards, and one "panameño" (I don't know the English for this). None of these students could make out completely what he was saying--just a few words here and there.
Julio Iglesias
I don't think that it depends of the native's country accent. I think that depends more of how the native vocalize it.I know people of my enviroment that is very hard to understand them, and when i have been in other places with diferent accents, I have found people easy and hard to understand too.
Please correct my English , I have wrote it like an english native would have done, I mean : De carrerilla y sin pararme demasiado a pensar donde he podido meter la pata.
I speak from experience that people from Panama are notoriously difficult to understand by many other Spanish speakers because of heavy use of slang and some major differences in pronunciation of some words.
This is hard to do, because if I can't understand someone I can't very well reproduce what is unintelligible to me. ¿Verdad? I might be able to find an interview with Julio Iglesias on Youtube. If I do, I will post it in this answer.
I found one.
