I think that conjunctions to the personal planets by the outer planets have the greatest impact upon one's personality and behaviour.
Mars and Jupiter conjunctions bring fiery energy to the native when conjunct personal planets.
Mars conjunct Mercury or the Moon can make one react with anger, impatience or frustration. Or it can make someone feel bullied or pressured and they may feel victimised.
If it is positively aspected, it can bring energy and ambition to the forefront and help one achieve personal goals.
Jupiter can bring enthusiasm and optimism and the native may be more open and socially engaged, with Jupiter conjunct their personals. Jupiter conjunct Mercury or the Moon may open the way for the native to be more open minded or more accepting than usual. They may be more interested in new experiences or learning new ways to cook or learning about different cultures or religious beliefs during a Jupiter transit.
Jupiter conjunct Moon or Venus may spur one to be 'open' to romance and more prone to take a chance on love.
But if you are asking about a 'complete' change of personality, I would say Uranus, Pluto or Saturn conjunctions to the lights or Mercury/Venus or Mars. Or perhaps the Ascendant too?
I had a good friend that I had known since childhood, walk up to me, and I didn't even recognise him, he had changed so much. He had gone through Pluto conjunct his Ascendant in Scorpio, and Saturn conjunct his Moon/Mercury in Libra. He lost 80 lbs, and his hair had grey streaks and he was only 33. He had a changed so much physically and in outer temperament....