The best and most versatile combo to roll is Soul of the Dark Knight/Lord’s Soul.

Soul of the Dark Knight, the set bonus from 2pc Bale, is non-negotiable on Great Sword. Rolling it on our GS allows us to get the set bonus while only wearing 1pc Bale armor. This is efficient because the Bale armor is Resentment-focused and designed to be paired with True Shadowbringer, but we’re not using that GS. It also just gives the most flexibility.

Lord’s Soul is pretty obviously the best group skill to roll as it’s really the only one that increases our damage, so hopefully that doesn’t require much further explanation. Getting this on our weapon lets us wear one fewer piece of AT armor. Although AT armor is relatively efficient, this is still nice because it allows us to more easily cram in additional set bonuses.

Other Viable Rolls

It’s still possible to make a highly performant GS set even if you can’t get the “god roll” of Soul/Soul. Bale bonus is still mandatory of course, so any mixed set that doesn’t have Bale on the weapon is going to need to wear 2 pieces of Bale armor.

Group Skill	Set Bonus	Suggested Set
Lord’s Soul	Doshaguma’s Might	Dosha / LS
Lord’s Soul	Ebony Odogaron’s Power	Ebony / LS
Lord’s Soul	Fulgur Anjanath’s Will	Fulgur / LS
Lord’s Soul	Gore Magala’s Tyranny	Gore / LS
Lord’s Soul	Omega Resonance	Omega / LS
Lord’s Soul	Rathalos’s Flare	Rath / LS
Lord’s Soul	—	LS Only
—	Soul of the Dark Knight	Bale Only

For detailed information on this subject, check out this guide by & Knuckles.