From my experience I will rank the Spanish accents with the strongest R sound. These are just the ones I get most exposure to so I can't say anything about an accent not on this list, because I have had little to no exposure to them.
This list is strongest on the top weakest on the bottom. Of course these are just general, not in specific regions within countries.
-Mexica
-Colombia
-Spain
-Argentina
Mexicans roll the r more often. Even when it's not at the beginning of a word or a rr within a word. I often hear the rolled r in words like alrededor, and I will hear the r rolled if its at the end of a word like in compositor.
Also I hear the Peurto Rican accent has the weakest r, which sounds like an L. Though I haven't heard enough of it to confirm that.
It took me 3 weeks to be able to make the rolled rr wound when I started learning Spanish. I ended up with a sore dried throat, but I got it. Now I'am at the ability to roll the R at the frequency of Spain speakers of Spanish. I think I will be close to Colombian in a few months.
I think like J.Rios. At least in Spain of course their is a clear difference between a soft "r" such as in "carta" and a strong one in "carro", but that "strong" sound likely would sound less "strong" to a Spanish learner that they could expect after be trained to get the strong "rr" sound.
One feature in Spanish is that consonants tend to sound quite softer than in English. In the case of "RR" sound quite often doesn't sound too different than in some british strongly rothic accents.
As a curiosity an to show how the difficulty of pronuntiation for English speakers to pronunciate the "rr" is a kind of myth listen to this famous video from the Sex Pistols (god save the queen). You can hear in second 30 how the singer perfectly pronounce a pure "rr" sound in the word "moron" in the second 30.
