Depends on the airline/alliance you are looking for, and sometimes which mileage program you want to use.

For oneworld airlines the best options are aa.com, ba.com, JAL.com and Qantas.com. You will need an account for the latter three. They each can search for a subset of oneworld airlines. The mileage program you use to redeem with, the airline you fly on and the website you use to search for availability do not need to be the same. For example I will often use AA.com to search for certain oneworld partners and redeem with BAEC avios.