Final Fantasy IV for PSP was the first Final Fantasy game I owned. I have completed the game six times, and the Lunar Ruins four times. Rydia is my favourite character in the game, and in one of my playthroughs I maxed out her stats by manipulating the random stat gains after Level 70.
I have played through the Interlude three times, and each tale of The After Years twice, with the exceptional of The Crystals tale, which I have done over ten times. I really enjoy the amount of playable characters it gives you and allows for so many different combinations (including several challenge runs I have completed; the twins only, Ceodore solo, Golbez solo, Rydia solo).
Final Fantasy for PSP was the next game I played... I've heard people say it involves a lot of grinding at some points; I got so hopelessly lost that on my first playthrough, I never even noticed (I've completed it once since then, and did find myself doing a fair bit of grinding).
I then got Final Fantasy III for DS, and I've found it very hard. Even now I'm stuck trying to get to the top of the Crystal Tower, and from reading online, I know it only gets worse from there.
I also bought Final Fantasy V for GBA. Final Fantasy V is tied with IV as my favourite in the series and I have completed it once entirely (including Omega MK.II, Neo Shinryu, and Enuo).
I really don't see what people find so hard about Neo Shinryu... well OK I do, but I've heard people say they level to 99 and still can't beat him... I beat him at level 55, on my second or third try, with not a single Dragon Lance. Not one. Just couldn't steal them from the stupid Crystal Dragons.
I really like the amount of customisation the job system gives you; I prefer it by far to FFIII's job system, which has that job-adjustment phase where you suck, and if you change from a non-magical job to a magical job, you have to rest up to restore your magic charges, which makes mid-dungeon changing useless.
Then I bought Dissidia Final Fantasy, though I'm not very good at it (but getting better), as it's not a traditional RPG. I have completed all ten Destiny Odysseys, Shade Impulse, and the quests for Shantotto and Gabranth. I've leveled Firion, Onion Knight, Cloud of Darkness, Cecil, Golbez, Bartz, Exdeath, Terra, Squall, Ultimecia and Shantotto to Level 100, and I have Warrior of Light, Garland and The Emperor at around Level 60.
Inward Chaos is HARD. I eventually got to stage 3 with the Onion Knight, and took out Garland. I failed a few times vs Shantotto and then ended up rage quitting vs Zidane. Fortunately I'd already collected all the good stuff (Moogle + Omega). There should be a userbox for completing Inward Chaos, just like for defeating Omega, Shinryu, etc.
I've started Final Fantasy XII but I'm stuck crossing the Dalmasca Westersand after completing the Dreadnought Leviathan.
I've also started Final Fantasy II, but it's not that clear on where to go, everything kills you, and I hate the stat-increasing system, so I may eventually get round to playing some more.
On this wiki, there's not a huge amount I can contribute, as most of what I know about is covered already. I have, however, added strategies for most of TAY True Moon bosses, because if I had to choose one area of expertise it would probably be that. I also go round correctly random typos or other mistakes (and occasionally make mistakes of my own =P) by pressing Random Page and reading the page I'm given. I think that makes me a WikiGnome or something like that.
The games I know most about are IV, TAY and V. I also particularly enjoy the Sealed Temple, so I have started a walkthrough covering it.