"In theory, fully upgraded Grandmas are the most efficient building a player can buy because they have a CpS based on the number of Grandmas purchased. That is, the Grandma's CpS rate increases as a square where other CpS rates increase linearly. However, the crossover point where Grandmas outperform the best other building, the Antimatter Condenser, requires purchasing so many grandmas (13408, slightly less if Portals are present) that it is impossible even via cheating, since the price would be much higher than JavaScript (which Cookie Clicker is written in) can store."
13.408 Grandmas...