I'm not as up on professional astronomers/astrophysicists, but I'd nominate Tim Ferris for the amateur category. His books are both entertaining and informative and reach a wide range of interests. And his PBS special was great. I know in the Chicago area it was viewed in many K-12 classrooms and likely sparked some new AA's.
That said, any number of people could qualify. How 'bout Dobson and the numerous "Sidewalk Astronomers"?
Maybe the most influential figure in amateur astronomy isn't an individual at all. Maybe it's more like one of the many local astronomy societies or the AAAA. Uh-oh, I'm throwing the net too wide for sure now! 8)
Sagan (RIP) influenced my formative astronomy interests most. His shoes remain unfilled in that respect IMHO.
As far as keeping amateur astronomy on an even keel towards steadily improving quality in all respects (for decades), as well as contributing a huge amount of guidance to amateur astronomers at large, as well as an unreasonable amount of personal guidance, Roland Christen and his company Astro-Physics are eligible for my vote.
I would most assuredly say the Niel deGrasse Tyson is the most visible professional astronomer today. He is seen on PBS on Nova Science Now and is the director of the Hayden Planetarium. He is very articulate and has been seen in the History Channel's "Universe" series.
Also, there is David Levy, whose writings have inspired thousands. He has reached non astronomers in Parade magazine articles.
There is also Michio Kaku, the Nikkei theoretical physicist that frequently features astrophysics in his programs on cable. His books are fascinating, too.
For longevity in Amateur Astronomy you can't really go past Sir Patrick Moore. He has authored over 70 books, is the presenter of the longest running BBC television series, The Sky at Night. He has contributed to the Astronomy community for over 50 years. Also, I favour Sir Patrick Moore because he was a Navigator for Bomber Command during WWII and since I have served over 20 years with the Australian Airforce I am a bit biased...sorry.