Personally, I'd start simple: brake pads.
Quick test: Use a hose to squirt some water on the brake rotor and pads (both sides of the rotor). Is the sqeak still there? Does it come back as soon as the water dries off?
If the squeak goes away with water, but then returns, I'd try removing the caliper and pads, cleaning everything up with Brakleen and make sure any caliper sliding points are correctly lubricated (use ONLY high-temp grease design for use around brake components; make sure the rotor and pads stay clean). Then put it all together. Make sure to pump the pedal several times to tighten up the piston and pads before driving off (I forgot this once; the first stop was thankfully safe, but heart-pounding).
