Best: Tactics. Because there's actually a game there and Ramza drinks Milk. Best 'numbered' FF for me would have to be 6, but at the end of the day, I find it increasingly hard to stomach RPG 'gameplay'.
Worst: Final Fantasy X-2. Technically, Final Fantasy II is the most unplayable of the lot, but X-2 is the worst simply in principle. It was completely unnecessary and signalled the start of a new, money-grabbing FF strategy that rehashed old ideas and digital assets and tied them together with a disinteresting and tired plot.
For the record: Final Fantasy XI should be aplauded because for an MMORPG it was actually reasonably good. Furthermore, it's the last example of Square actually doing something adventurous with the brand, and probably the biggest leap of faith in the history of the company (perhaps Spirits Within was a bigger leap. But FFXI wasn't ****.) It simply shouldn't have been a numbered game though. Why wasn't it 'Final Fantasy Online' or something?