First of pardon the english i am from Denmark
As mentioned before I think only the Ultramarines would be able to reform into a Legion again, because they are the only chapter who has the industrial might to support a legion force. According to a couple of different HH novels they numbered aprox 250.000 marines during the heresy, also their sphere of influence was somewhat bigger during those times so the influx of new recruits was bigger.
My second bid would be the Space Wolves because they also have a large industrial base i the Fang. Heard some rumors sometime ago what they even may have a functional STC which would be a great asset for a legion.
Dark angles and black templars could make what i would call a semi-legion numbering around 5-6000 marines each if they combined all their in the case of DA unforgiven chapters and all the crusades the Black Templars Has.