I like hovering just either side of 'normal', slightly wide (135) or slightly long (210, both on 4x5). I have a 300, but like Mike (post #3) it just comes along for the ride most times. Besides, I've only got a 300mm bellows draw, so even with raking my back out I can't focus closer than about 20', which is where a lot of the things I like usually are. I also like going super-wide too, I had a 90 (again, on 4x5) but just couldn't quite find a 'groove' with it, now I'm considering a 75, so my 300 may get passed off since there's only so much room for things that don't get used... but then comes the thought: "What if I ever get an 8x10?".
On my 645 kit I like the dramatic angles of the 35mm (which is why i'm looking at 75's for 4x5). Again, that's a wide-normal lens set, 35, 55, 75, the only exception being the 150 (comparable to the 300 on 4x5, except it actually gets used because I can actually focus it!).