IMO, the best set of starters are the Sinnoh ones. They all evolve into solid, useful dual-types and each one is distinctly different but they're all well-balanced. Every other gen has at least one starter that's mediocre at best.
My favorite set of starters though are the Hoenn ones. They're not very well balanced (Swampert is far and away the best of the evos and Sceptile is hampered by its shallow movepool). but I just like the designs.