I asked similar questions
here.
The DA 55-300 comes highly recommended. What I noticed however is that for its new price you can actually buy 3 used lenses and still have some change left. Even used, it's probably costlier than the FA 100-300 plus Tamron 70-300 and while it may be better than each (arguable in the case of the FA), it doesn't have the features that both add to the table - in particular the 1:2 magnification at the long end.
If money is no objection and you want one lens, go with the 55-300. If you want to use the lens on film as well, or you don't want to spend as much, check the FA or other older lenses like the Tamron SP 60-300.