Very good question, and knowing me, a very long answer... I don't have a single favorite band, but rather several.
Band name: Mournful Congregation
Genre: Funeral Doom Metal
Location: Australia
Notes: One of the oldest Funeral Doom bands and one of the best, early material is beautiful and raw funeral doom, The Monad of Creation remains to be the best funeral doom album ever made (and my favorite album of all time in any genre) and The June Frost exhibits a slightly more modern sound that I simply cannot wait to hear what could possibly be next (not to mention that The June Frost is the best album ever to listen to while it is snowing outside).
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Band name: Therion
Genre: Death Metal (early), Symphonic, Operatic, Progressive metal (Recently)
Location: Uplands Vasby, Sweden
Notes: One of the first metal bands that I got into. Their sound changed with each of their albums. On Of Darkness, they were straight up Death Metal, but became more progressive on Beyond Sanctorum and started to experiment with Symphony Masses: Ho Drakon Ho Megas. Lepaca Kliffoth and to a lesser extent Theli they became more of a symphonic thrash metal band, with their operatic sound fully developed as of Vovin, Deggial and Secret of the Runes. They are unique among the symphonic bands in that while using operatic and symphonic elements, they keep the metal parts HEAVY... the riff of Ginnungagap is one of the heaviest riffs of all time. Progressive and power metal elements started to find their way in Therion's sound in the Sirius B/Lemuria era and were expanded in Gothic Kabbalah. Christofer Johnnson is the sole original member and the mastermind behind Therion... he has, on several occasions, fired everyone in the band and replaced them with new members. The most recent purge was because the other band members were taking Therion's sound further from his vision (arguably, Gothic Kabbalah is different in sound than all their previous albums)... so it'll be fascinating to hear where their next album will be like.
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Band name: The Gloominous Doom
Genre: Death/Thrash with grindcore, ska and power metal influences.
Location: Fleetwood, Pennsylvania
Notes: Home town band that I've seen live at least 100 times. They are releasing a new album in May, by the time that album is released I'll know every note of every one of their new songs, because they vary their set list so often and I see them so much. Also, their songs are extremely catchy! Drummer Jer Stoyer got me into local music when he was in Six21, an alternative metal / punk band.
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Band name: Killswitch Engage
Genre: Metalcore / Melodic Death Metal
Location: Massachusetts
Notes: My favorite "mainstream metal" band. Before I got into true metal in 2005, I dabbled in some nu-metal such as Korn, Slipknot and even Linkin Park. As of 2005, I listened to a lot of metalcore as I delved deeper into the underground. By 2007, most of the bands I first listened to that were mainstream disappeared from my MP3 player. Those songs got old and tiring since the music is uninspired and weak as opposed to timeless classics such as Black Sabbath. But... Killswitch Engage is the clear exception. Not a single bad song and Alive or Just Breathing is one of my favorite albums of all time.
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Band name: Black Sabbath
Genre: Heavy Metal / Doom Metal / Classic Rock
Location: Great Britain
Also known as: Heaven and heck
Notes: I'm not actually that big on the Ozzy era Black Sabbath. Sure, their debut album set the standard for heavy metal and the next several albums up until (but not including) Technical Ecstasy are downright essential... but the real gold of Black Sabbath's output came after the so called "Prince of Darkness" was kicked out and the best singer that ever lived came on board: Ronnie James Dio! Heaven and heck, Mob Rules and later Dehumanizer and The Devil You Know are four of the best albums ever released. The Tony Martin era of Black Sabbath is also excellent, my favorite albums from that era is Tyr, The Eternal Idol and Headless Cross. Cross Purposes is also a good album.
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Band name: Eiffel 65
Genre: Europop / House / Techno
Location: Turin, Italy
Notes: Now known as Bloom 06 with two of the original members of Eiffel 65. Their sound also varies between each album. Nevertheless, this band, especially their first two albums, are major sources of nostalgia. Blue came out at the end of 1999, I started to download from Napster in January of 2000 (before it became a crappy pay service), so Blue was one of the first MP3s I had. Their second album, Contact!, is one of my favorite non-metal albums of all time and one that I can listen to through and through without coming across any sort of crappy track. Those sort of tracks, do actually exist on Europop, such as Playstation (song is boring)... but none on Contact! and none on their Italian language 3rd album Eiffel 65. Bloom 06 was a new incarnation of the group, minus Gabry Ponte and they have since released two albums: Crash Test 01 and Crash Test 02. Basically, they have modernized the Eiffel 65 sound and matured it with even better production:
http://www.youtube.com/watch?v=pffTr5vCsdM
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Band name: Sister Hazel
Genre: Alternative / Southern Rock
Location: Florida
Notes: Another of the nostalgic groups, their classic songs such as Happy, All For You, Champagne High and Concede... among several others... found their way on my early MP3 players back in 2000, 2001... in 2006 they released an album called Absolutely, which I now regard as their best album yet and one of my favorite non-metal albums of all time.
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Band name: Toad the Wet Sprocket
Genre: Alternative / Folk Rock
Location: California
Notes: Same deal with Sister Hazel. Not a single bad album, some songs have a sort of melancholic atmosphere that captivated me in recent years, and other songs are pure nostalgia taking me back as far as 1994.
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Band name: Electrasy
Genre: Alternative
Location: Great Britain
Notes: I only have their CD In Here We Fall and some tracks from various other albums (they only have 3), but they've managed to do something that no other band has ever done: Write a song that was never topped as my favorite song of all time, Cosmic Castaway. This song appears in what used to be my favorite movie of all time, Titan A.E. (favorite movie is now Avatar, but Titan A.E. is still one of the best movies ever made)
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Honorable mentions: Judas Priest (Heavy/Speed Metal), Abyssmal Sorrow (Black/Funeral Doom Metal), Moonspell (Gothic Metal, Black Metal), Goatwhore (Blackened Death Metal), and probably a zillion others.
