Buz Mcgrath. Unearth was the first 7 string band I listened to and then i saw a Steve Vai clip where he was demonstrating his 7 and that sealed the deal
John Petrucci, Jeff Loomis, Frederik Thordendal to name the top three. It was Mr. Petrucci that set it in motion with A Change of Seasons. Loomis sealed the deal
I don't own a 7 string yet but I still want one badly Slipknot cause they sometimes use drop A Tosin Abasi no explanation thera haha and actually tuning down to b I liked the low sound
I can say although I would listen to korn I had no clue they were using 7 strings. Once I got into playing music it became more of unearth and korn that made me want one, mainly because I couldn't learn their songs without one. Then I discovered down tuning and blah blah blah the list then switches to petruccie, Messhuga, korn, periphery, trivium, born of Osiris, and so much more but it I got a shitty 7 string but I felt better because I had one. But I sold it now I'm on to getting my Agile =D
I listened to 7-string bands like Nevermore, Meshuggah and Dream Theater, but what actually prompted me to get a 7 of my own was my friend Anthony in VMOD.
For me it was Dino who first got me wanting a 7. I loved Korn and their tone but always thought it was super muddy for my personal tastes, tho it worked wonderfully for their sound. But it was Dino and his tone that showed me 7 strings don't have to be muddy and are good for not only sludgey stuff but also for precise percussive playing; he was also the reason I went with the EMG 707, which I still use to this day!!! Back then it was the only pickup that gave the 7 string tuning the clarity I like and is still hard to beat in my opinion.
Another band who got me wanting play a 7, was a quasi-death metal band called Embodyment. They had one good album called Embrace the Eternal(everything else after that sucked), after seeing them live in 1998 and purchasing that album shortly thereafter I absolutely had to have a 7 string!! I would say that not only were they the main determining factor in me getting a 7 string, they are also still to this day one of my biggest musical influences on both my guitar playing and drumming. BTW, to all those reading this that have never heard that album, check it out now!!!!!! That album was way ahead of it's time!!!