No isolation, best fidelity: Grado (any model, but preferably the 125 or better).
Good isolation, very good fidelity: M-Audio IE10 buds.
Best isolation, good fidelity: ExtremeHeadphones EX29 (the latest version only). (Some headphone amps are a bit puny for these if you like to listen too loudly.)
Good isolation, decent fidelity: Sennheiser 280.
Lousy isolation, mediocre fidelity: Sony 7506.