Sometimes you get confused while improvising.
Sometimes you just don't know where in-game Synergy Encyclopedia is (hint: bottom center on settlement screen).
Sometimes you need something open on a second display.

Here we are.
Your cheat sheet for all above and then some.
Since you usually can't get BOTH STR and INT to values high enough at once.
And you really need to specialize in either.
Unless you're running Poison centric meme builds, of course. Poison sucks bad as of today.

It is implied that you're running 6 skill slots, which means no slot adjusting Difficulty settings (or weapons, or artifacts).

So classification below assumes:
- All STR - at least 6 skills are STR based
- Mix STR - STR is prevalent but there are not enough skills to fill all slots
- All INT - obvious
- Mix INT - obvious
- All Artifacts - no skills with this synergy, only artifacts

As most Synergies have 6 as Synergy power cap by default, Synergy Breakthrough sources (from artifacts and constellations) might be necessary to reach maximal power (which is at 11 unique sources of Synergy).
Sources of ALL Synergies boost (or) ALL Synergy Breakthrough boost are not included. Only Synergies themselves.

Base game (of version 1.4.11) and all 4 DLCs are covered here.
Artifacts you unlock with Achievements are included but not specifically listed.

Dark (6 STR, 3 INT, 11 Artifacts)
Puncture (7 STR, 4 INT, 9 Artifacts)
Pierce (6 STR, 4 INT, 13 Artifacts)
Poison (8 STR, 0 INT, 9 Artifacts)
Wind (7 STR, 2 INT, 10 Artifacts)
Death (8 STR, 0 INT, 9 Artifacts)
Assault (8 STR, 2 INT, 11 Artifacts)

Annihilate (5 STR, 4 INT, 11 Artifacts)
Shatter (5 STR, 4 INT, 11 Artifacts)
Blood (5 STR, 4 INT, 13 Artifacts)

Holy (0 STR, 9 INT, 13 Artifacts)
Fire (0 STR, 9 INT, 10 Artifacts)
Lightning (0 STR, 9 INT, 9 Artifacts)
Frost (2 STR, 7 INT, 11 Artifacts)
Torrent (1 STR, 7 INT, 12 Artifacts)

Trap (4 STR, 5 INT, 10 Artifacts)
Summon (4 STR, 5 INT, 9 Artifacts)

Time 12
Space 15
Godspeed 15
Vitality 11
Scholar 12
Feast 14
Defense 13
Physics 15
Magic 13
Dexterity 15
Destiny 18
Greed 13
Demon Soul 8
Mortia 7
Tarasaya 7