You will get a lot of different opinions. Personally, I feel like beef rib bones and beef shank bones are the safest thing for them to chew. I have been using beef rib bones and shank (marow bones) for years. I have my butcher cut the shanks about 3/4" thick. I use the meat for human consumption and clean the bones well. I have had at least a dozen yorkies and our boxer gnaw on the shank bones for years and have had to throw only one away due to wear. When the bones get nasty, I wash them in the diswasher.
The shank bones are really neat because you can rub peanutbutter or cream cheese around the inside for extra chewing pleasure.