Orokin Derelict Survival and Defense are great for most of the relics. There are choice Lith and Meso (T1/2) relics in the A-rotation and then a very high chance of some Neo-relic in B-rotation and Axi in C. Other places drop them mostly in B or C rotations, depending on the mission tier. I'd say Derelict is generally the best place to go for them, though there are few particulars where Derelict isn't amazing. Some list I'd seen on Reddit:
Spoiler: Best relic farm locations
T1 (Lith):
- Tikal (Earth) (Excavation, Rotation B)
- Cervantes (Earth) (Excavation, Rotation B)
- Kiliken (Venus) (Excavation, Rotation B)
- Martialis (Mars) (Excavation, Rotation B)
- Wendell (Phobos) (Excavation, Rotation B)
- Orokin Derelict Defense (Rotation A)
T2 (Meso):
If Meso B2:
- Orokin Derelict Survival (Rotation A)
If not Meso B2:
- Lillith (Europa) (Excavation, Rotation B)
- Valefor (Europa) (Excavation, Rotation B)
T3 (Neo):
If you want Axi V2 on the side:
- Despina, Neptune, (Excavation, Rotation B)
If you want greater chances at T3 relics
- Cholistan, Europa, (Excavation, Rotation B)
T4 (Meso):
- Hieracon, Pluto, (Excavation, Rotation B)
Note, the preference for Excavations is because Excavations go through Rotations the fastest and thus you get the most key rotations for your time invested if you do them efficiently.