Most difficult or best? They're not necessarily the same thing.
Best would have to be BG2, with BG1 coming a close second. BG2 is just an awesome game, endless options, excellent replayability, great interaction. BG1 though was incredible in terms of immersing you in this epic story. I still remember how exciting it was to finally get into the city of Baldur's gate.
They got the "feelgood" feeling right with BG1.
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Memnoch - Custodian of the Order of the Holy Flame 