Let's think of the effect that any of the choices would have had on his actual Bulbasaur, Charmander, and Squirtle:
Ash chooses Bulbasaur--his actual Bulbasaur lives a peaceful life defending the secret garden from outsiders.
Ash chooses Charmander--his actual Charmander dies in a rainstorm and is consumed by Spearows.
Ash chooses Squirtle--his actual Squirtle continues to wreak havoc with the Squirtle Squad, is eventually caught by the authorities, and is put down to protect the citizenry.
So, Bulbasaur is the only safe choice for the pokemon o_o;
Now, for Ash:
Bulbasaur--Ash has strength against Brock and Misty, can survive Lt. Surge, and will have enough pokemon to survive by the time he gets to Sabrina.
Charmander--Ash loses at Brock and ends his quest.
Squirtle--Ash beats Brock, vs. Misty is up in the air, and definitely loses against Lt. Surge and his quest ends.
So, basically, Bulbasaur > Squirtle > Charmander > all other pokemon > Pikachu.