I'd say it's out of Teach, Smoker, Eustass Kid and possibly Coby.
the last enemy will be teach, no question about it. if you look at his crew they are the mirror image of the strawhats.
Going with the majority to say Blackbeard
Someone mentioned earlier about the fact that he has the initial "D" in his name and I'm guessing that that's very important in finding/discovering "One Piece".
But I feel that Luffy will fight the others after defeating Blackbeard. You know, a friendly rivalry match between Shanks, Coby... etc...
Oda wanted Buggy to be the ultimate enemy of Luffy...so i think this will be his last fight. But on the other hand we don´t know how much his decision will be influenced by the magazine-staff
i say overthrowing the world government and whatnot will be the last thing he fights for
WG I think
since BB is only after one piece and I expect Luffy to get it(at least I believe)
but after that, BB will loose interest and probably wonder around as yonkou
hence only one more wall, WG
I would be sweet to see him to fight law or Kidd. I think Croc or Moria will be fighting him towards the very end. I don't think they'll get a final fight like boss fight just the last person to oppose them before Luffy dies.
i think it will be shanks(luffy said the he will beat shanks and his crew and give his straw hat back) or the whole marines
Blackbeard will be Luffy's final 1on1 fight. The World Government will fall shortly afterwards, but will be a combined effort by Luffy and allies.
I would say Coby, Smoker, Gorousei Eustass or Law and slightly Blackbeard/someone else.
The reason is because i think since it took the strawhats 10 years (of manga time) to reach the New World (mid point to raftel) i think that within those 10 years Luffy will want to beat Bleackbeard up as soon as he gets the chance, he probably won't chase him but when he gets his chance he won't give up! Someone different will probably end up as the last enemy to the strawhat, probably the Gorousei or someone completely we haven't met yet!
Davy Jones will be the last enemy . He will come and kill them all like the insignificant pests that the yare AHAHhahahah
My vote is for Colby....oh or possibly Sabo
Somehow, I don't think BB is last opponent. I get all the facts, (Bittman -.- that was a large fraction of my time used on MH) but i just have a hunch. And it WILL not be Roger, or i will stop reading OP. Roger's death was magnificent, and meaningful, him being alive would ruin Oda's goodname for me. What i think is possible is either Kong, a fleet admiral, The GUY talking to Doflamingo, or a Tenryuubito that is strong (u never know, its Oda) or someone else that has royal blood, ( a red-line ruler perhaps?) or something