Fire Temple, because it felt like the longest with five semi-large floors and yet it never got boring with dozens of unique obstacles, some of which were actually challenging. Many of its rooms have a kind of 'steam filter' effect, and I was also lucky enough to get the original version's music.
Funny thing is, before buying Twilight Princess I saw a mention of the Goron Mines and then a mention of the Fire Temple which led me to believe they were two seperate dungeons with the much harder Temple coming later on. Considering only a portion of the Mines was an actual 'Fire' dungeon they could have easily revamped the Fire Temple to be bigger and badder than ever. A missed opportunity.
Funny thing is, before buying Twilight Princess I saw a mention of the Goron Mines and then a mention of the Fire Temple which led me to believe they were two seperate dungeons with the much harder Temple coming later on. Considering only a portion of the Mines was an actual 'Fire' dungeon they could have easily revamped the Fire Temple to be bigger and badder than ever. A missed opportunity.