I'm thinking melee weapons that have some kind of overt gimmick, or otherwise unusual properties. Examples:
Keratinos grows and gains +2~3m range when you do a 12x heavy
Ghoulsaw has 100% followthrough
Caustacyst globs force enemies open to finishers
Gazal machete gains Corrosive damage after Djinn casts Fatal Attraction
Sarofang has a vortex on heavy slam
That said, I don't want to consider signature passives, as they're already documented. Incarnons are also well-documented, so I don't think they're as interesting to document or discuss.
Kestrel inflicts a forced ragdoll on enemies when thrown, a fact I relied heavily upon in my very early Warframe days (you could even use it to kill some bosses by ragdolling them into insta kill pits)
If you're fine with gimmick for a class of weapons, Daggers are able to open enemies up to finishers with heavy attacks.
The Dark Split Sword is interesting in that depending on the stance used, it's either a Heavy Blade or Dual Swords. I wish you could toggle between the two modes mid mission though.