CO(Condition Overload) vs PPP(Primed Pressure Point)
CO has the advantage with high SC(Status Chance) and/or forced procs on Stances vs enemies that take 4+ hits to kill. So for most enemies below lvl~100 or so PPP is preferable.

The Faction(Smite) and 60/60Cold are flex slots.
Good options:
Quickening - +DPS and faster slam animations.
Faction(Smite) - High damage increase, but faction-specific.
Vicious Frost - A little damage and semi-bursty damage bonus through Viral.
Voltaic Strike/Shocking Touch - For Corrosive damage, giving a big boost to damage vs Ferrite Armor.

Riven stat priority: Range ? CD = AS > Dmg > CC > Elemental
Range is a great stat for melee, but doesn't bring a quantifiable damage increase, hence why it's "?" in relation to the others.
Riven replaces [Organ Shatter], [Primed Fury] or [Primed Reach] if it has one of those 3 stats, or Primed Smite Grineer if you want a generic build.

Meta builds for melee and the rough lvl-range where they perform best:
Burst DPS - For lvl0-100.
Sustained Slash DPS - For lvl100-2000
Sustained Toxin DPS - For lvl2000-9999

As for how to identify which weapons are good, it's a combination of their stats with how good their Stances are.
Spreadsheet with Stance data: https://docs.google.com/spreadsheets/d/1FIJV02-mFmeKH8OC6k4lKycVJ95680B5kML_Pmj8TW0