John and Sergio at Global Fish and Pet have given me great advice, pretty much about everything in this hobby. They've told me more than once that I don't need something that would have resulted in a good sale for them. They're not afraid to tell you that they don't know something if they don't. It's pretty hard to figure out 'one' thing they've told me, or helped me with there's been so much.
Toni is the owner at Planet Fish. She's very knowledgeable about pretty much everything, and is very willing to share her knowledge, or answer questions.
Kenny at Aquatic Center is very helpful. He's willing to special order things for his customer, and seems to really know his stuff.
This might have been a little more generic than you're looking for, but I've gotten so much advice, and help from the guys at Global, and from Toni at Planet Fish that like I said, it would be difficult to come up with one example.
Brian