Curry is probably referring to
this old thread.
Let's itemize some of the battles. I don't know if I can choose one as "the best", since all of them have their merits. I haven't included many of the standard "dungeon quest" encounters or run-of-the-mill bandits:
The first wolf you meet outside of Candlekeep (seriously)
Shank and Carbos (you have to admit they're funny)
Tarnesh (a real wake-up call at the Friendly Arm Inn and the scourge of a 1st level party--even tougher than the first wolf you meet)
Karlat (dwarven Fighter at the Red Sheaf Inn in Beregost)
The Flaming Fist mercenary who mistakes you for a bandit (south of Beregost)
Neira (Nashkel Inn--that Hold spell can be deadly)
Nimbus (Nashkel--I didn't expect him at all the first time I met him when my party was limping back from the Nashkel Mines)
Vitiare (the magnificent Thief who robs you in the gambling tent at the Nashkel Carnival--good luck trying to catch him)
The "Amazons" outside the Nashkel Mines (Zeela, Maneira, Lamalha, and Telka--very tough)
The assassins at Gullykin (Drakar, Molkar, Halacan, and Morvin--not especially tough, but nice equipment and an attitude that's so annoying, it's entertaining)
The bounty hunters sent by Rieltar to meet you at the Cloakwood Mines (Drasus, Genthore, Rezdan, and Kysus)
The five Ogre Mages in the southern sector of Baldur's Gate
Larze the Ogre at the Blushing Mermaid (dumb but very strong!)
Slythe and Krystin (Undercellar)
The party hired by the Iron Throne to kill Sarevok in the Undercity who will also attack you (Ahvin, Gorf, Carston, Shaldrissa, Wudei, and Haseo)
The adventurers you meet in the wildnerness area with Korax the Ghoul and all of the basilisks (Kirian, Peter, Lindin, and Baerin)
The snobbish Amnish adventurers south of Nashkel (Sendai, Delgod, and Alexander)
Vax and Zal (who really is the fastest dart thrower in the west)
Shoal the Nereid and Ogre-Droth (on the coast south of Candlekeep--definitely a reload the first time you meet them)
The Maulers of Undermountain (Gretek, Wilf, Arlin, Caturak, Pargus and Nader) at the Helm and Cloak Inn in Baldur's Gate
Honorable Mention: the xvarts who attack Hulrik's cow, Arabelle