For me, it has been Mora's Hand; because I love Fire themes.
While I would prefer it to be a ranged/mage most times, currently we don't really have any Flame runes for Staff, Gauntlets, or even Bow. So I've been using Sword or Sword/Shield.
But what I love is how everything about the setup can be Fire/Flame. With Enkindled on my eapons, they glow with Fire, the Rune ability is Flame, and ALL three of the Pact skills are Flame/Fire. But also, they are actually all useable in the rotation. Flame Thorns > Run in, Flame AoE > Fireball any shield or tough ones; while slashing with Fire sword and stabby fire rune. And more often that not, pulling smite recharges the ones on cooldown and makes them useable again without long waits.
So it's fire everywhere, and things burning/melting everywhere.
Anyways, this made me curious on which other themes/Pacts other people have fun playing, and why (even if it's not the strongest).
PS. Of some of the others I tried, I didn't like some being more situational, so they didn't get much use etc. And there was often a disconnect between the Pact, and the weapon/combat game play.