The gameboy games, despite their names, were not Final Fantasy games at all. They were part of the Mana series. Squaresoft kind of poohed the screwch on naming those games.
If you like FF2US, you may wish to give the translated Japanese FFIVHT a go. FF2US was identical to the second release of FFIVJ, which uses the EasyType ROM. FFIVHT is the HardType, and let me tell you, it's a real challenge. And it's the only of the three original SF/SNES cartridges to contain Cecil's Dark Wave move.
FFVI (3US) was by and large very enjoyable, though I would give anything to have a HardType version of it. In its original form it can be cheesed so easily. Many people didn't know, but you can get infinite EXP Eggs. Made leveling a lot easier for sure.
FFVII wasn't so much a mess as the graphics limitations of the time just made it look that way. The only change I'd make is for most of the materia to be more lenient in how much AP is required to level them. It wasn't leveling that was a grind. The grind came in leveling up your materia so your characters were of actual use outside One Trick Pony™ abilities.
FFVIII had very good combat mechanics, great graphics for its time, but its fatal flaw comes in the storyline at the end of Disc 2. This game has the most ghetto storyline of all of the Final Fantasies, outdoing the craptacular that is FFX-2 by a wide margin. Also, expect the card game AI to cheat - often.
FFXI's gameplay was more nostalgic than revolutionary. It retained the ATB system of FF8 mixed with the Limit Break mechanic of FF7, while utilizing the modern day (at the time) version of FFV's graphic style. The biggest challenge of this game? A head-bangingly harsh LV 1 Perfect Save challenge. Expect to wear out a PSOne or two with this challenge (or a couple DVD drives if you use an emulator and don't use ISO images of your real discs).
FFX was the first to have voice acting as a mainstay in its gameplay. With the graphic capability of the PS2, even in its early stages of life cycle, the story really did unfold pretty well (with a couple hiccups that could be forgiven). The downside to this game? It has just about every cliché you can think of in it, and getting from Point A to Point B took foreeeeeeeeeeeeever on the ground. This game would have seen much needed difficulty enhancements with an ATB system instead of its turn based combat system. It's got a few tear jerker moments in the story though, something of a rarity in video games.
FFX-2. Dress Up System. Um...yeah. I got about 20% through this game and said forget it. Feels like I'm playing Final Fantasy Barbie Fun House. Direct sequels can be cool. This was not.
I'm skipping over FFXI because it's an abomination and needs to DIAF.
FFXII had what I read to be a very well done storyline. Unfortunately for myself and likely many other players, the controls just could not be tolerated. Because of its new RTB (Real Time Battle) system, the dual inverted controls made for a veritably lousy play experience. So much so that I didn't even get past the first Field Map area before I promptly uninstalled the game from my system.
FFXIII is pretty. Very pretty. It has its moments of Slowdown Salad, but I expect that with such intricately rendered scenes (though most of the slowdown comes from too many player models onscreen at once). Storywise it's OK. The combat system uses a mixed ATB/RTB system in that it has the ATB command queue, but has no "Wait" option available, meaning you must make quick decisions. This is partially offset by the AI which controls your NPC comrades. Purists will hate how you cannot control (directly) your teammates, but with the design SS chose to use for the battle system it was the best compromise possible. The game's major downside is that until you get to Chapter 11, it is completely linear. We're not talking linear with a few things to explore as side quests, but instead Lunar: Eternal Blue type linear, where there is literally no exploration whatsoever. Until chapter 11 I felt like I was being led on a dog leash and spoonfed every last bit of detail on how to act during battle. Chapter 11 offered much needed exploration, but all you get is Hunt missions, not anything really story related in terms of side quests. In a nutshell, FFXIII went for All Glitz, Little Glamour. Unlike previous FFs, replay value on this one is absolutely nil. Thankfully I got it at $39 new while it was still $59 everywhere else. This game could really use an achievement system like Star Ocean 4: The Last Hope had, and a New Game+ of ANY sort.