My list of frustration:
- SF Dragon (and to a lesser extent the SF Hydra)
- Cordi fighters from WoD
- anything that can cloak (Strakhas, Chimeras, Pegasi)
- the Horus-X from Awakenings does deserve a mention, tho these days it doesn't give me as much trouble
- AF Kato, depending on the mod (piloting it in ASW is often difficult too, due to lack of shields)
- Damn near everything in Dimensional Eclipse

Basically anything that's horrendously maneuverable, tiny, or hard to shake.
Sorthaks also get a mention for being made of turrets and extremely tanky, but once I learned
The Way Of The Dumbfire I found they didn't bother me much anymore.