Most influential is hard to say because most influential
doesn't necessarily mean that the particular artist made it big. They are the artists that influenced most of the people that are still around today, or at least still remembered.
For instance, Elvis is gigantic. Everyone who is anyone knows who Elvis is, yet he was heavily influenced by artists such as Dean Martin, Ray Charles and Big Joe Turner, who, lets face it, would never be considered to be bigger than Elvis.
Anyone know of Huddie Ledbetter, A.k.a Leadbelly? A young man by the name of Kurt Cobain was deeply influenced by him and years later went on to form Nirvana.
Elvis, of course, went on to affect many other influential artists such as The Beatles, Led Zeppelin, Freddy Mercury.
Anyway, influential or not, The biggest artists over the last 30 years (and I'm talking 70's, 80's and 90's because this decade isn't over yet) would have to be: (in order, oldest first)
The Beach Boys
The Rolling Stones
Boyz II Men
Nine Inch Nails
Marilyn Manson (love him or hate him, he has raised quite a few eyebrows)
I think I will stop there. I know there is probably a lot I missed, But I am only going off by who I
think is influential. I also don't necessarily like everyone I wrote down (Although, I'll admit I like most of em...) and as you can tell I've left a lot of today's more popular "artists" off the list because quite frankly I do not believe they have enough integrity to join those that are on the list.
That's my 5 cents anyway.
There have been tons of influential artists in the last 30 years some of which had good influences and some bad. They include
Madonna, Michael Jackson, Nirvana, Billy Joel, Ray Charles, Stevie Wonder, The Eagles, Toby Keith, Eminem, Elton John, Brittany Spears, Ozzie Osborne, and many many more!
You may like their music or not like it and there are a few in there that I don't care for but you can't say they haven't effected politics, pop culture, fashion, movies, and more. Like I said some of their influences have been good and some have been bad.