A carseat is the safest. If you were to crash or go through turbulence, there's a good chance that you can't hold onto your kid or you will smush them into the seat in front of you. (just like in a car).

On my adoption group there are some car seat on plane fanatics. They've posted videos of what can happen on a plane. I'm not a fanatic so I haven't seen the videos for myself but apparently it scared the pants off these people enough that they use them. Personally, having a squirming 2 year old on my lap for 2.5 hours sounds hideous!