Mewtwo:
* Pros: The most powerful Pokemon in the game, with incredible stats and devastating Psychic-type moves.
* Cons: Catching Mewtwo without a Master Ball is incredibly difficult, and it's only available after completing the game.
Articuno, Zapdos, or Moltres:
* Pros: These Legendary birds are strong and provide valuable type coverage, making them excellent additions to any team.
* Cons: They are relatively easy to catch without a Master Ball, and their individual strengths are not as overwhelming as Mewtwo.
Other Considerations:
* Snorlax: While not legendary, Snorlax is a powerful and bulky Pokemon that can be a valuable asset in battles.
* Mew: While not available in the original Blue version, if you're playing a ROM hack or have access to a modified game, Mew is a legendary Pokemon with the ability to learn any move.
My Recommendation:
While Mewtwo is incredibly tempting, I would personally use the Master Ball for Articuno, Zapdos, or Moltres. This gives you a strong Legendary Pokemon with great type coverage, and it's a more practical use of the Master Ball, as catching them without it is more manageable.
Ultimately, the choice is yours! Choose the Pokemon that best fits your playstyle and will make your journey through Pokemon Blue even more enjoyable.