The sketchiest airline I've probably flown is Garuda Indonesia, and it wasn't that bad. I flew from SIN > CGK, CGK > DPS, and then DPS > KIX. Their A330 from DPS > KIX was much more comfortable than the UAL 777 I flew home on. It wasn't scary at all. The city of Jakarta was another story.

I also flew EVA a few years ago, if you consider that dodgy airline. I was supposed to be on the nonstop LAX>SIN, but SQ cancelled the flight and they put me on BR to TPE instead. Since the vast majority of other SQ passengers got on the SQ flight to TPE, there were about 20 people on BR, so I had an entire row of a 744 to myself. Most sleep I ever had on a plane, and the service was great.

I don't think there's any airline I wouldn't fly, as they all are probably equally safe. I suppose I probably wouldn't fly any back-woods Africa airline that's still flying Caravelles, but reputable airlines fly to just about anywhere I need to fly to.