Final Fantasy got its name because it was the assumed last effort of the ailing development studio Squaresoft. Not only did the game end up saving the company, but Final Fantasy as an IP has remained one of the biggest JRPG series until today.
The series has repeatedly tried to reinvent the wheel and itself, commonly with mixed results and receptions. Because the polls on this site only allow for up to 10 options, I've had to decide to put only the better entries as individual options while the lesser games are lumped together in "Other". Main series means all numbered titles, including the two MMORPGs, as well as direct sequels, so Final Fantasy X-2 and the two Final Fantasy XIII sequels are acceptable too.
What's your favorite main series Final Fantasy game?
Bonus question: What are your hopes and expectations for the upcoming Final Fantasy XVI?