$$$ not being an object... a custom tune. No 2 cars are alike, and hence a custom tune will ensure a safe, well performing vehicle.
Handheld tuners are great if you're not planning on making serious power, or are aggressive on your tune. The fact that they're about $300-$400 doesn't hurt either.
Custom Tunes can go from $400-$1,000 depending on the setup and tune required.
Also, custom tunes will always net more HP with a competent tuner. It won't be double or some astronomical amount, but 15-35 hp differences aren't unusual.