For me:
UK Domestic (full fare): BA from LGW or LCY
Domestic (lo-cost): Virgin Blue. In the UK, easyJet.
European: KLM
International: Malaysian
Puddle-jumpers: Aurigny
BA at their best are the best in any category - but at their worst they are awful. The Houston-Detroit leg of a LHR flight particularly sticks in my mind
13Alpha