The only dogs I would avoid would be ones that are not always so good with smaller, non-canine pets - greyhounds, border collies, terriers etc. Not because of the dogs themselves, but just because of our lifestyle and requirements from a companion. It's not fair to get a dog that doesn't suit you.
Also, I would probably never get a Rottweiler. I know this is just plain prejudicial, but a Rottie chased me up the street barking and growling when I was a little kid, and then I had another HUGE one lunge to attack me when I was about 17, and they just scare me. I've met many wonderful Rotties and I know they are just gorgeous dogs, but I wouldn't own one because like I said, they just scare me, and you don't want a dog who's going to scare you just because it's a Rottweiler - it doesn't make for a good partnership!