Generic advice: hard case, and airport security friendly. Where you go from there is up to you.

Avoid: soft cases, and complicated closure mechanisms.

Edit: one more thing. Make sure the combined weight of the case + bike + spare parts and tools if any is under 23 Kg / 50 lbs. Some airlines will charge oversize and overweight separately. I have the trico ironcase it's right on the limit because the empty case is so heavy, A pedal wrench packed in the case will push it over the limit.