I love my Pennant. Easy red crits, guaranteed slash proc on heavy attack, and has the passive on increasing attack speed on heavy attack kill.
This is the build I use, but if you don't have a riven would probably slot in Corrupt Charge or another Gladiator mod for the set bonus
Most of the scythes in the game, especially Reaper Prime, are also very good with this type of build (look for stances with guaranteed slash or weapons with mostly slash damage)
In addition to this, Tatsu Prime is also another great choice and another two-handed Nikana. Just don't use these weapons for light attacks with their stance, it sucks.
Daggers? They aren't exactly a heavy attack weapon but enemies hit by a Heavy Attack from a dagger are opened up for Finishers, assuming they aren't finisher immune. I forget if their heavy attacks come with a forced slash proc.
I just wasn't sure if the heavy attack multipier was any good on daggers. I do like modding them for finishers, you end up getting access to certain buffs like Arcane Trickery/Ultimatum that you normally don't get with most builds.
A Dokrahm heavy attack build (Dokrahm has highest base dmg) paired with exodia brave makes one of the best infinite energy utilities in the game. The trick is to ignore attack speed and rely on heavy slam attacks + rolls to cancel the animation. I believe it still often procs the headshot bonus. (I might be wrong)
Anything with high critical stat, and a high base damage (preferably weighted toward slash damage) is going to wreck shop. Gram Prime, Nikana Prime, Syam, Azothane, Pangolin Prime, Tenet Livia are all good choices, and if you're willing to include daggers in the list, Karyst Prime is a beast. You also have some pretty good options for zaws with the added bonus of zaw arcanes.