Mjolnir → EM damage

Best vs: Sansha, Blood Raiders
Also use in Electrical Abyss (enemies have reduced EM resistance).

Inferno → Thermal damage

Best vs: Serpentis
Good general-purpose choice if you're unsure.

Scourge → Kinetic damage

Best vs: Guristas
Often strong against many Caldari-style NPCs.

Nova → Explosive damage

Best vs: Angel Cartel
Also use in Firestorm Abyss (enemies have reduced explosive resistance).

T1 Abyss quick guide:
Electrical → Mjolnir (EM)
Firestorm → Nova (Explosive)
Dark → Use the damage type the NPC is weakest to (no universal bonus).
Exotic → Scourge (Kinetic)
Gamma → Usually use the NPC's natural weakness (no resistance penalty).

Damavik/Kikimora/Vedmak/Leshak → Inferno
Sparkneedle/Needlejack/Decimator/etc. → Nova
Watcher/Warden/Preserver/Upholder → Scourge
Nullwarp/Strikegrip/Anchoring → Scourge
Devoted/Loyal/Faithful/Fanatic → Mjolnir
Anything starting with Gistii/Gistum/Gist → Nova