No particular order.
Iron Maiden- hearing Hallowed Be Thy Name on the radio back in 2003 officially made me a metal head.
Rush- I used to hate Rush when I was a little kid, particularly Geddy's vocals but something clicked when I watched Rush in Rio, particularly Natural Science. Opened my eyes to complex music that I still enjoy today. Still one of my favorite bands of all time.
Blind Guardian- first riff and verse of Majesty turned me into a huge power metal fan.
Dream Theater- another band that took forever to click but once it did, opened up the flood gates for other bands that I never really appreciated before, like Opeth and Tool.
In Flames- While not my favorite band anymore in terms of death metal vocals, I had a pretty hard time getting used to growling until I heard Jotun.
Def Leppard - the first band that I was ever obsessed with when I was younger. Pyromania still stands as one of my favorite albums ever.
Styx- 70's arena rock had always been something that was drilled into my head by my parents, and Styx to me still stands on top of that time. Dennis DeYoung was my favorite vocalist for many years.