The sin is defintely not the fastest anymore, maybe the night walker but not the sin.
With Cygnus Knights, Heroes, and Resistance, the explorers are "slow".
You want fastest? Cygnus Knights. Probably the Night Walker or the thunder Breaker(Pirate)
Bossing? Idk since Big Bang, but I suggest the Night Walker or Mechanic(Resistance)
over-Powered? Mechanic. These are the most over-powered, ksing class on MS.