Graves is the best generic ADC. He has the highest burst, highest AOE, increased durability and even an on-demand attack speed bonus tied to mobility. That's all you could dream of in a generic ADC.
However, he doesn't have the range, tank-shredding, late-game scaling mechanics or poke that some other ADCs have. His utility is also pretty weak even though it is fairly unique. All of these traits can make an ADC a much better pick than Graves in certain games, but he is your best go-to generic pick imo.
Edit: Caitlyn is probably the most reliable in-lane and pretty good late-game, but has one of the lowest carry potentials throughout the longest part of the game(mid-game).