mqstout: Not sure why people are praising 12 in here. It was the worst in the series until 13 came out.
X (perhaps X-2) was the last good Final Fantasy game, and,
mechanically speaking, possibly the best of any of them. Storywise, 4 or 6 is best to me. 10's isn't too bad in the story department though.
As for "corridor", X was no more corridor than those prior. You can backtrack at almost any point (and there are reasons to), and once you get the airship, there's a lot to do spread. Now 13, that was truly a corridor experience until a tiny spread at the end.
From a game mechanics perspective, FF10 has some issues.
* Physical attacks scale cubically with strength, while spells scale only quadratically. As a result, in the long run physical attacks end up stronger than spells. (And unlike FF6, spells don't have high enough constants to make them strong throughout the game.)
* The most powerful weapons ignore defense, but only for physical attacks. These end up being mandatory for post-game, and effectively make attack magic pointless. Plus one of their abilities, Break Damage Limit, is basically mandatory.
* The effects of Agility (or is it Speed? I don't remember the exact stat name) are not continuous. I think I may have read something about Agility being pointless past 170, even though the cap is 255. (Then again, I don't like having a stat that affects how often someone gets to act; said stat ends up being too powerful.)
Sachys: I would say 7, 8, 9 or 12 would be better starting points.
Don't start with 8; that game is very strange mechanically, and the game isn't balanced; if you know the system, it's too easy, and if you don't, it can be too hard (especiaily if you take the time to gain a few extra levels, as the game has level scaling).
(FF2 isn't a good starting point either; while there's no level scaling (and no levels in the first place), the game isn't exactly well balanced, and the game even hides a rather important strategically significant mechanic, one that should not have been hidden, from the player.)
MajicMan: 4-6 are great. For many players, these represent the high point of the series. Great characters, story lines the job system the Active Time Battle System all get introduced in these three and perfected.
The job system was actually introduced in FF3, with FF5 refining it and adding the ability to learn and equip an ability from another job.
FF4 and FF6 don't use this system at all. (Also, I would argue that FF6 and FF9 have the worst ATB implementations, if you don't count the buggy implementation found in the WSC/GBA versions of FF4.)