Lahm.
Both Zabba & Micah Richards (at his best & fit) were proper defensive fullbacks imo. Not Pep style wingers/midfielders disgused as fullbacks (Alvez & Marcello are the best in that regard) but I'd rather see Zabs/Micah defending, than any of the main players today, including Walker.
We have just seen this weekend, Trippier, Clichy, both playing for top 4 teams, sidestepped & left for dead by the slightest trick. We have seen Walker, Bellarin, made to look absolute mugs time & again, Chelsea don't even bother with fullbacks, Milner & some random no mark at Liverpool, etc etc.
Years ago, in wide open football, people like Tony Book used to have to face up to people like George Best running at them for 90 mins. Tactics didn't stop it like in today's game, these guys had to figure out the impending trick & stop him. Or he was away. And they did.
I wonder how many can do that now.