Using the ringdb "Hall of Fame", these are the highest listed with deck title (with type in bracket if it isn't obvious from the name or heroes) for each combination. That doesn't mean they're necessarily the most powerful, however. For a given sphere combination, what heroes/archtype do you think would be the most powerful?
Using the abbreviations D - leadership, L - lore, S - spirit, T - tactics
DDD LeBoromir, LeDenethor, LeFaramir (Family of Leaders)
DDL LeDenethor, Hirluin, Erestor (The Rapid Charge of Outlands!)
DDS Amarthiul, Elfhelm, SpEowyn (Rohan, meet Dunedain)
DDT LeDenethor, Sam, TaBoromir (Now it's time for... Deorwine) [sword-thain Deorwine, build him up]
DLS Dain, Bifur, Oin (Quad Sphere Dwarf Deck *COTR deckbox*)
DLT Sam, LoPippin, TaMerry (Stings Like a Bee) [hobbit secrecy]
DST LeDenethor, Arwen, TaAragorn (The Grudge) [TaAragorn superhero]
LLL Mirlonde, LoPippin, Rossiel (Become the Forest *CotR deckbox*)
LLD LoAragorn, Damrod, Amarthiul (Dunedain Trappers)
LLS Mirlonde, Rossiel, Galadriel (Victory-a Secret)
LLT --none listed--
LST Galdor, Arwen, TaBeregond (Everything* Costs 2)
SSS Arwen, Caldara, Eowyn (I Will Survive)
SSD SpGlorfindel, SpMerry, LeDenethor (All Signs Point to Yes)
SSL SpGlorfindel, Arwen, Elrond (Forest Gump) [Vilya]
SST SpGlorfindel, Galadriel, TaBoromir (Seastan's Boromir)
TTT TaEowyn, Hama, TaTheoden (The Rohirrim Arrive)
TTD Hama, Legolas, LeFaramir (Blacken the Sky *CotR deckbox*) [combat deck]
TTL TaEowyn, TaMerry, LoPippin (Eowyn Esquires and Ents!)
TTS Gimli, Thalin, SpEowyn (Beorn's Path 3 -- Escape From Dol Goldur) [core set only]