Who is considered the most knowledgeable Marja Taqleed right now, and who is the second-most knowledgeable Marja at this time, according to most of the Ahlul Khibra and the learned Shia?
Bismihi ta'ala
From what our respected 'Ulama in the Hawzah say, it is a bit difficult to specifically pinpoint who exactly is the "most learnt" from among our esteemed living Maraji', may Allah prolong their lives.
On the topic of selecting a Marja', it is best that you refer to the rulings and following the correct criteria for your selection. It is not a difficult process, and it should not take a lot of your time.
With prayers for your success.