For me fire dungeons represent the incarnation of hell in the zelda universe filled with fiery demons and beasts set to a volcanic backdrop. Link is only one mis-timed jump from being scorched by the flowing lava that hinders his progress. What i like most about these dungeons is there atmosphere and the isolation and danger that i feel for link. IMO the fire dungeons are the most evil and demonic looking in any given zelda game and thats why i love them.
Bear in mind that i'm talking about the five 3d home console zelda games because i do not play the 2d or handheld games. Anyway just so you remember the five bosses in these dungeons are volvagia, gohma, fyrus, scaldera and ghirahim.