Watch the following cool of video that compares all the voices of Rocket Raccoon.
Depending on whom you ask, Rocket Raccoon has at times been thought to speak with an English accent as the character was inspired by the Beatles song, "Rocky Raccoon" as well as being written by UK writers Dan Abnett and Andy Lanning.
As we see from the video, Rocket Raccoon has also spoken with more of a slight Brooklyn accent, which James Gunn and Bradley Cooper went with in the Guardians of the Galaxy movie.
I'm not sure if Gunn ever said why he went with an American accent over a English accent, but I've always read the character with an English accent.
From the video, I do lean more toward Greg Ellis' version of Rocket Raccoon from the Avengers: Earth's Mightiest Heroes animated series as well as the Ultimate Marvel vs. Capcom 3 video game.
With the success of the Guardians movie, I'm guessing Rocket's American accent will be a permanent fixture for the character from here on out. I wonder how general audiences would have reacted to Rocket speaking like a Brit?