I know that if I say "never" I'll end up having to use a particular airline again sometime, but I've managed to avoid the following:

1. Lufthansa, where on our one trip with them (honeymoon, yet) my husband and I were involuntarily bumped from a flight to St. Petersburg and then the flight we actually took arrived an hour late. Yes, I've had worse happen, but this was after paying premium prices on an airline that used to advertise its reliability. That was 2003. Haven't taken them since, despite the fact that I've been to our offices in Munich twice since then, and could have taken them on my 2004 trip to India.

2. Continental, after they decided to award only 50% EQM for people on bargain airfares. I know they eventually backed down and started awarding 100% if you booked through the Web site but they lost almost 3 years of my business- during which I made Platinum on AA and my husband and I went to Europe twice a year. We flew CO once in that period- to Cleveland.

3. Heathrow. My boss is in London so it's hard to avoid completely, but since a nightmarish connection at LHR early last year, I booked our next 2 European vacations to avoid LHR, even going to the point of taking CO to get to Edinburgh to avoid a London connection. (Never say never, right? And I made Platinum on AA anyway.) And I plan to continue to avoid LHR whenever I can until they stop making life hell for the average Coach class traveler. I'm not holding my breath.