My top 10 is probably always changing, but right now I guess it would be (in no particular order):
Sally Mann: I love her earlier work, the large format photos of her family. Some of it is just fantastic. Her recent stuff bores me (body farm stuff).
Harry Callahan: He's just got the kind of weird eye I like. I love that portrait of the tiny guy at the far end of an alley.
Margaret Bourke-White: Many classic images. She had a slick way with composition.
Weegee: I'm turned on by anybody who does LF hand held, and I love Speed Graphics.
Mark Citret: A contemporary LF photographer. I like his urban landscapes. There is a great story on his website about how he got a poor critique on one of his photos, and later it was purchased by Ansel Adams.
Paul Caponigro: A lot of my stuff looks like his stuff, at least the landscapes. We have a similar eye.
Ansel Adams: I know many folks find his stuff boring, but I've seen a lot of it in person, and I like it. It may be sort of "calender work", but heck, it's some of the original calender work. Also "The Camera", "The Negative", and "The Print" are my holy books.
The Bechers: Large format industrial stuff that is made with amazing precision. When I get to see their work in person it's stunning.
Diane Arbus: Some of her stuff is great, some of her stuff is kind of crappy. I like the folks she chose to photograph: freaks instead of the beautiful people. For some reason I find her inspirational (you know, except for the suicide), and I love Rolleiflexes.
It's hard to limit the list to just 10. How can I leave out Berenice Abbot, Edward Weston, Minor White, Huger Foote, and so many more. That's why I say my list is always changing.
How about least favorite photographers? My least favorite photog would be Annie Lebowitz. I'm just bored to death with her stuff. William Wegman would be a close second. I actually like some of his stuff, but geez-louise!!! Take a picture of something else besides Weimerreiners (spelling?)!!! I could almost like Anee Geddes, until she did this Celine Dion thing.
EDIT: Whoops! How could I forget Charles Jones?! My favorite vegetable photographer. He did his photography in the late 1800s, but wasn't discovered until the 1980s. That pepper may be Edward Weston's most famous image, but Charles Jones was doing it decades before. This guy makes fine art out of lettuce, cauliflower, and brocolli.