It depends what you want to achieve. If, as someone else said, you just want to prevent noise fatigue, then good ear plugs do the trick. Some people find them to be uncomfortable for long periods, in which case I'd suggest something like the smaller of the two types of bose headphones, as they are very good, but not too bulky. The bigger over ear ones probably work better, but I find the smaller bose's do the trick. The Bose's come with a two plug/one plug adptor for different airlines - which is actually more important than people realise, as the second pin controls volume (I think).
For short haul flights, I carry some in-ear bud style SHURE headphones, which are pretty high end, give good sound, and have many different types of ear buds to suite different ears. In my experience, on short haul, they do the trick, although its a hassle taking them in and out to talk. I think SHURE now do a better set, with a button on the cord, which allows you to switch to hear people speaking to you, so saving you have to take them out.
For long haul flights, I definitely prefer to use the Bose's. On one very long flight, for the ultimate in isolation, I used, ear plugs, with the bose's on top, and a very high quality light mask (which blocks 100% of light). Total isolation. Pure Bliss.