National Assoc. For The Advancement Of Young Entrepreneurs
The three classes that excell at current 45 +0 patch are Summoner, Blade Dancer and Force Master.
Most of the people will agree with summ and bd but not FM simply because it's actually hard to learn, but that doesn't change the fact that it's super brutal on this patch with good player on it.
The rest of the classes are all perfectly viable and good player will be able to do well on all of them. Certain matchups are hard but it's to be expected from a mmo with a fighting game combat system and that won't change much with a 50 patch.
A lot of people think that once they have bigger hp pools and HM skills they will do well, SPOILER they won't :D. Players that are very good now, will be very good after 50 too and players that are bad will stay bad as long as they make excuses.
For example sin vs summ it's a hell for a sin now but even at 50 it's still a counter matchup, then you may need Blade & Soul Power leveling, that's just how it is. But still if sin player goes god mode he can win.
All classes are on the same level, balance wise there is no true best and worst class. You should play whatever class you have modt fun with, the assassin has a similar playstyle to the slayer, where you can OS people with one combo, although sins arent very tanky and have the stealth skills. If stealthing isnt yours blade master might work for you, too.