I say every proc is situational and not really the best. Luna has lower activation rate than Rend Heaven but at least guarantees you bonus damage since every unit has some stats in def and res.
Rend Heaven has higher activation rate than Luna but against certains units like Magic vs Berserker or Physical vs Sorcerer, it's useless.
So this is more on preference of the player, Either you want consistent activation or consistent bonus damage.
Lethality has extremely low activation rate 1/4 of your skill but (and I feel people forget this sometimes) bypass things like counter, magic counter, aegis, and pavise. It's a gimmicky skill but it does the job.
Sol is the same as Luna in activation rate and is pretty much meant for the tanky plug unit. it combos really well with certain characters and skills. (like Hinata and Counter)
Vengeance, same activation as RH. Combos with certain characters and skills. Risky but with certain back-up like high evasion or dual guard, It will come in clutch. Astra half your skill, just does more damage and not much damage against high def or res units. I was wrong, Aether is the best proc skill. Half your skill, heals and always guarantees you do more damage. Hail Ike.