I'm with Gaear on Stirges. They're the most common random encounter, they come in large groups and are small and hard to see. You also get little reward for killing them as well. Also Will-O'-Wisps. Immunity to magic and high AC makes battles against them drawn out and annoying.
Will 'o' wisps and snakes. If I encounter snakes while traveling it takes them forever to reach the party... And yeah stirges and giant tick, cause they always seem to hit... And drain blood... And Cutberth for steppin in to soon to save Iuz's @ss
Magic Missile works great on the Will-o-the-wisps- that or get really lucky hits with any enhanced weapon...
Salamanders and their fireballs. Note that I dont recognize raise dead possibility and in this case load last before-fight-save to try different tactics.
My strategy is to only put the rogue (with Evasion) in range of the fireballs. Everyone else has Far Shot.
I'm going with who said the bugbears. Specifically those douchebags who just don't care about life and only want to punch your caster. A close second is anything that casts fear. In fact, fear is just my most hated spell.
They're not really that bad. Casters have all sorts of ways to avoid harm like Stoneskin, Mirror Image, Mage Armor, Shield... etc etc... Personally what I like to do when dealing with them is use Ghoul Touch on them and then follow it up with a coup de grace from my caster! Making them die in the most humiliating way possible!
I had fighter (tank), paladin (tank/heal), ranger (I find his ranged attacks very useful at many situations, for example to take bosses with spell resistance which are busy with summoned animals, weaker enemies and casters when fighters are busy or too far, and also as wizards defender. In original game, could not open just a few chests with open lock skill, but wizards knock spell did. Hide and move silently were also good enough when needed), wizard (because can learn many spells from scrolls and be a good crafter) and druid (summon/heal/extra attack with longspear behind fighters). I just remember, that in original game, salamanders pissed me most, mainly at fire temple (not fire node, but somewhere in temple dungeon level 2 or 3) where they had support of many fire elementals and were on end line. I more like to fight with many bugbears than with salamanders, especially as end line support for other enemies. Maybe its off topic, but other situation that pissed me much, was suprise attack of two adventurers from hommlets inn (these recruitable NPCs whos names I forgot) with nice support, after I killed Lareth and was not prepared for another bigger fight.
Very easy to avoid Noble Salamanders fireballs: Lesser Globe of Invulnerability. Game over for salamanders, you win.
In line with Gaear's original post I do not fear bugbears - I just find their behaviour exceedingly annoying on principle. The reason a bugbear can tumble through to attack the mage is because your party tanks are engaged in a fluid melee thus preventing your mage using the fireball spell the bugbear might want to prevent. Rather than using defensive magic - I protect the mage by using my tanks defensively across a bottle-neck and fireball the cluster of bugbears that forms. The result is that bugbears adopt a strategy that ensures battles are fought to their disadvantage.
I still say Gobbos, but I have to voice my hatred for those BugBears that, like said earlier, will run the gauntlet just to try and trip your caster.