No, I do not agree.
I simply think Mario 64 is better, especially if you compare them both to the times they came around. I enjoy Mario 64 more than I enjoy Galaxy when I play them today, despite the controls being tighter in Galaxy and the camera being better (and obviously, the graphics being vastly superior).
Thing is, Galaxy feels way too linear to me. There's no exploration to be had whatsoever(well, hardly anyway), and the areas you actually walk around at tend to be very small. In Mario 64, the areas are big and you can go your own way. Each level feels more like a sandbox than a line. Also, I generally don't gripe about things being too easy, but Galaxy made me do so because I think the game was just way, way too easy. People keep saying "Yeah, clearing the main quest is easy, but some of those hidden stars are really tough"... But that's not my experience. Sure, a few of them took some tries to get, but generally nothing felt very hard in Galaxy, to me.
So no. Galaxy is not the best 3D Mario ever, in my opinion. Galaxy 2 may well be, though.