I am driving poor Sergino Dests thread off topic.
Let me say it here instead: if I was to build an optimal line-up I'd probably have Alba on the LB.
My reasoning being the optimal line-up would be focused around the best team I watched - Peps Barcelona. The Busi-Xavi-Iniesta midfield is a no-brainer, as is Alves and Messi. You could argue that Abidal was great for the balance at LB and that it was key for the team to work so well, and I'd maybe agree, but if you want a LB that is great offensively as well I would prefer Alba to Roberto Carlos for sure, in that kind of team. Marcelo could be good, but his style wouldn't complement the other players as well, as he wants the ball on foot and is even more unreliable defensively.
My initial claim in that other thread was that Alba maybe could be considered the best LB the sport ever had, and I see no reason to back down on that. There are of course many candidates.