If we are talking about the "best executed" launch as opposed to generally what was the best launch, I'd say it's a tie between the NES launch in North America and PS1.
With the NES, it was more of a roll out than a launch. They were entering a market where console gaming was essentially dead and retailers didn't want to touch consoles with a ten-foot pole. I believe even Atari turned down the exclusive distribution rights for the NES in the U.S. Nintendo, instead, had the balls to roll out the NES in New York before taking it across the U.S. The rest is history as they say.
The PS1 launch was also one of the best executed IMO. Sony wasn't known for gaming back then and it was still essentially a two horse race between Sega and Nintendo. Countless other electronics manufacturers tried to cash in on the video game market with hardware with abysmal results. Not only did Sony successfully launch hardware in a market previously dominated by Sega and Nintendo, it also had an impressive launch lineup and a steady flow of launch window games. Sega stumbled with the Saturn at a time when they could have taken advantage of Nintendo's choice to stick with expensive cartridges and floundering third-party relations. Sony not only fostered strong third-party relations out of the gate, but offered competent, easy to develop for hardware. The hardware was also relatively easy to find on store shelves.
I think you could argue that the Wii had one of the best executed launches as well, considering it was going up against the HD twins but managed to set the world on fire with an affordable price and mass market appeal that resulted in prolonged console shortages. The only reason I would say it wasn't the best executed is because they couldn't meet demand. The same goes for the PS2. The demand for the PS2 was insane and the shortages were even more crazy.
Just talking about generally great launches, I think the Dreamcast is hard to beat. It came in much cheaper than its predecessor and had a great launch line up and supply.