Lemme rank my favorites:

• Mei - Genji and D.Va can't block her freeze, I can't aim so she's easy to play, I can pull off all sorts of SICK MOVES with her wall and ice-block, and her name starts with M

• Moira - DPS healer, I consistently get a lotta medals with her, no aiming skills needed, name is an anagram of Mario and starts with M

• Mercy - Ez pz and effective healer to play as, good in lockout elimination because she can rez, her name starts with M

• Torbjörn - Pure sex appeal