Going to try to keep this restricted to rock/metal/punk and not mix my other tastes (blues, jazz, classical, ethereal, world music, etc.) into the mix.
Xandria - Somewhat gothic-flavored rock. Immediately brings to mind bands like Lacuna Coil and Evanescense but the comparisons do them no justice - it just puts you in the neighborhood of what they do. They manage to be different from the flock though. They have a style all their own, lyrics with lots of heart and diverse and beautiful music to match. One of my very favorite bands.
Motorhead - Raw, pure rock'n'roll. Simple, straightforward, packed with attitude. There is nothing else quite like them. Note quite metal, more than punk.
Masters of Reality - Sabbath influenced hard rock with a southern twang. When their first cd came out we always said that they were like Sabbath mixed with Lynyrd Skynyrd. Since then they evolved into a more lazy southern sounding band with an amazing flair for cool, fun little rock songs.
Iron Maiden - What can I say about Maiden? Intelligent, driving metal with amazing guitars (dueling solos, amazing harmony lines, technical perfection), great vocals (both Paul and Bruce - didn't like the guy that took over when Bruce left but then again I had stopped caring about Maiden by then), and amazing live shows. I've spent more of my life listening to Iron Maiden than any other band.
Opeth - Dark and beautiful. Consumate musicianship and a style that runs from light and beautiful to dark and crushing and hits every last point in between.
Jimi Hendrix - The most amazing musician I have ever heard and a wonderful songwriter as well. 'Nuff said.
Rolling Stones - Blues and soul fused with rock. Rock'n'roll incarnate. Second only to the Beatles.
Other bands that I truely love but don't feel deserve to be listed in my favorites:
The Ramones, The Beatles, U2, REM, Cream, The Guess Who, Black Sabbath, Elastica, Evanescence, Judas Priest, Carcass, old Morbid Angel, old Entombed, Dismember, The Crown, Faith No More, old Metallica, old Megadeth, Belly.