In my youth, when I could train myself like a rat, I completed Ninja Gaiden 1, 2, 3 on the original NES.

Thank you.
Although, to be fair, it wasn't exactly my first puzzle game. It's just like anything else, you get better with practice.

Getting 99 points (max score) in the Desert Buss mini-game from Penn & Tellers Smoke and Mirrors. That takes 41 days, 17 hours, 15 minutes and 6 seconds... and there is no pause or save...

My main accomplishment with FF5 is that, on an otherwise normal run, I beat Omega when it first appeared.

Incidentally, when playing FF5, I like to play with the following rules for normal playthroughs:
1. No changing class to Freelancer or Mime. (Makes the end game more interesting.)
2. No throwing money at the enemies. (Really, that skill is way too powerful, especially considering how easy it is to get (it's even the Samurai's job command).)
3. No using the Blessed Kiss mix on enemies. Using it on allies is OK, and actually a useful strategy that doesn't completely break the game. Using it on enemies, however, allows you to inflict Berserk on enemies immune to it.

I have also done some silly things in that game, including getting one particular endgame boss to attempt to spam Self-Destruct, which kept failing because the enemy had no MP remaining.

I once beat The Adventures of Bayou Billy on a rental. I also beat Super Ghouls and Ghosts a couple of times. Those seem to be regarded as pretty tough games.

Here's another one I just thought of.

In Might and Magic 2, I defeated the Cuisinarts without winning any other battles first. That's a lot of experience for a first victory (enough to raise everyone to level 20+).

I always change to freelancers and mimes for the final bosses, mainly because I spend the last few hours of the game grinding for AP to get the perfect combo of skills.
For that run I believe I went with a Mime with Double Cast, White and Black Magic; a Freelancer with Dual wield, X-Attack and Magic Sword; a Freelancer with Summons and Time Magic; and something else.. I can't remember what the last one was.
I always used to spam Throw Gil (or whatever they call it) when I first started playing but not so much the last few times I'd played.
I didn't much like Chemists (except for Revive, which can be quite useful)

Now I want to play FF V XD

Edit: Actually, looking at How Long to Beat 24 hours wasn't a bad time for a speedrun. I think I even 100%'d it in that time too..

-Playing supreme ruler ultimate edition, with Belgium in a 1936 campaign, and having conquered the USA, South America, Middle Africa and The entirety of Europe before 1970 while getting reelected to parliament every time again.

Medium difficulty levels for military and economy, hard for diplomacy.

That took quite a bit of exercise and a whole load of keeping many saves and reverting whenever something went wrong ...


Honourable mention:

... Playing a lot of Morrowind as a young lad who couldn't understand English ... Chaotic evil playthrough ...

I think the only game I was actually ever really good was Captain Claw. I could never find/get all of the treasure (in the later levels, early ones are easy), but I usually came really close to 100% per level and made a speedrun once with no deaths up to the end boss, where I slipped to death two times before defeating him. Literal buckets of sweat is a word here.

Yes actually, but I never got around to doing it and it's been years since I've played it now.. I'm sorely tempted to go find an emulator but I've got enough games that I've never played before to play and if I start playing SNES games again I'll never stop..

How? You couldn't have played it legitimately for that long?! o.O

Not me, no. But there have been Desert Buss marathon events. That's an achievement and a half.

Desert Bus for Hope? It's been a while that I checked on how they are doing, and my mind was blown just now: last November they raised $683,720! Wow. That monotonous bussing though...