I really like my 7.5 gallon SS Brewtech conical. You can do 4.5 gallon batches in it with no problem. I have both it and the half barrel models so I can do a couple different batches back to back.
Whatever one you buy, make sure to get a butterfly valve on the bottom of the cone. They are way easuer to keep clean than a ball valve. I have ball valves on the racking ports of both fermenters but they are the one you can take apart to clean. I am thinking of eventually putting butterfly valves on the racking ports as well.