I always enjoyed the lower-level play in AD&D and the like, where every monster you encountered was a challenge and each choice of spell, weapon, or what-have-you really meant something. I think quite a few games loose something when you get to the higher levels.
Because of that, some of my favorite monsters are the lower level ones - the same kind that just become quick grease-stains later on.
I remember a particularly memorable adventure where our freshly-minted characters had been sent to clean out an abandoned tower that was now a breeding ground for stirges (am I remembering the name right? The big mosquito-bird combo thingees that'd attach to you and drain away until dealt with . . . )
We battled the aforementioned nasties, plus the cheerfully insane cleric who was breeding them, and feeding captured villagers to them, thinking he could train them into a loyal force. In the end, we tossed him into a room with a bag full of his "pets" and spiked the door shut. Heh-heh!
