Lots of threads on this. Do a search and you'll find a whole bunch of replies.
I know the pain though. Most frustrating thing in the world.
Is it just bent and rubbing in one spot as it spins? If so a shifter spanner tightened around the rotor (with a bit of cloth between) can sometimes wiggle out the bend with a bit of persuasion. Don't go too far though. Better less than too much.
Otherwise it could be a myriad of things such as, pads not aligned, brake housing not aligned/centered, brakes in need of bleed, dropout worn on one side among others.
Best thing is to put a bit of white paper or a torch on the floor underneath the bike, spin the wheel and look through the brake assembly as the rotor spins with the white paper/torch in the background shining back up through.
This will silhouette the rotor in the assembly and make it easy to spot any warps or mis-alignments.
If the rotor is too far to one side, just loosen the bolts on the brake housing and move it move a mm or two.
Best answer I can honeslty give, and I'm not trying to be a smart ass here, but just buy shimano brakes.
Hope this helps.