My mind quickly went to the uniqueidentifier (GUID) data type. It may not be fair but I think of it as my least favorite. The reasoning is more of a pet peeve. Most of the time there’s nothing wrong with the uniqueidentifier data type; however, it makes me cringe if it is the clustering key on a table when an INT would do just fine because it ends up wasting disk space.
On this topic, Jeff Moden had a really great presentation for our SQL Server user group. He has a rather contrarian take and interesting findings on fragmentation in practice. It’s a lengthy and advanced talk, but definitely worth the nearly 2 1/2 hours.