The Ropers against the New Phlan Liberation Front in Pool of Radiance.
First time I remember I was deeply offended that the trolls will rise up again with
full HP and then they slaughtered the party who's last words were "this is not fair". But I soon learnt that standing on the troll corpses made things much easier. Hiring a couple of Heroes as meat shields also helped a lot.
4x99 Berserkers in Bard's Tale, and then returning to the Review Board my party kicked in the wrong door and "You meet death itself in the form of 1 Hobbit".
The Captain of the Guard in Bard's Tale. WFT?!? He critically killed my character?
Fighting mirror images of your own party in Ultima IV.
All the epic fights against armies of kobolds and orcs in Pool or Radiance. Especially the ambush at Sokol Keep.
Slaying the dragon in Dungeon Master. Round and round we danced around the central pillar until finally there was a puff of smoke and when the smoke cleared the battleground was covered with neat stacks of dragon chops.
Using hearing instead of sight to keep track of dragons behind illusory walls when "waltzing" them in Chaos Strikes Back.
Fighting some mages in Bard's Tale 2's beginner dungeon. They were at 70' at kept summoning wolves, and it took what seemed like hours to finally defeat them.
The 3 Cuisinarts in Might&Magic 2. They sliced, diced and chopped my party into mincemeat.
Frist time I met Crazed Natives in MM2.
The first fight in Curse of the Azure Bonds where the party sides with the Thieves' Guild vs the Fire Knives. CoAB was my first Gold Box game, so this fight was special to me.
Defeating the Beholder Corps in same game without using the Dust (braindead AI helps).
Buffing the party in Icewind Dale and then walzing through the entire Frost Giant clan like a hot knife through butter.
Fighting the four Wizards of Thay in BG1 with the Sword Coast Strategems mod installed. It took my party more than a day to finally kill their leader.
Moving wounded and disorderly through The Underdark in BG2 and being surprised by a large band of bounty hunters. Too bad the WTF moment can not be replicated when replaying the game.
Final battles in Pools of Darkness. Not for the fainthearted.
Dragon King and his minions in Nahlakh. Only time I saw a character take so much damage that he slid all the way across the screen.
Monster of 1000 Eyes in Wizardry 7. Thought it was impossible, but then I realized Resurrection can be used in combats.
All these were great because they were hard and/or memorable.