There is no website that gives you the level of detail and information that a GDS can give you. From routing options, number stopovers permitted, cancellation and change penalties to fare families, added options and upgrades, the GDS remains king.
At Major, we have created a GDS emulator that works with all 4 major GDS input commands. Search, hold seats, create PNRs and much more. In fact you can do virtually everything possible on the GDS except issue the ticket!