Worst defeat I've ever inflicted? That would have to be the 7 warp openers and 30 star destroyers deployed simultaneously in PBW game Shattered Hopes.
For something not involving stellar manipulation, there was one battle in the RTV Dertran game where I was attacking through a warp point with 200+ ships against another 200+ ship armada. I completely obliterated the enemy force and lost only 12 ships. Yay for an enemy that kept using missile ships for far too long
. Sadly, the ONLY minesweeper in my fleet was thoroughly crippled and took two turns to get to the head of the repair queue.