as they say, there's no replacement for displacement. So, the biggest hp & torque gains will be to do an engine swap with a higher displacement engine.

second or equal to engine swap in terms of impact in hp/torque will be forced induction either by installing a turbo intercooler or a supercharger. assuming you have the appropriate gear and installation is good, you can realize 25-50 hp gain. some even claim 50-75 hp gain.

other mods will just be supplementary in terms of hp/torque gains. i.e., cylinder head works, increased forced induction boost, air intake/exhaust optimization works, installing lighter flywheel, regearing, etc... but all of these will at most give you 10-25 hp gains.