Hmm, I don't really know...
As each time I'm getting better at videoganes, previous victories which I thought were difficult may now be much easier.
Anyways, I still remember when I beated Boeser in Paper Mario (for the Nintendo 64). I remember it was very hard because, being that my first playthrough, I mostly raised my HP and FP, without caring about the Badges, thus losing important attacks and upgrades, among other things. I also had the issue that the battle became extremely hard after Bowser recovered 30 HP... But I managed to beat him because in one of my dozens of battles against him he didn't recover =D
Also on two other games on the Nintendo 64, I got very happy: when I completed Super Mario 64, which was a lot harder than I thought (took me five years to beat the game!) and when I completed Jungle Japes in Donkey Kong 64 (do you know how big the game is? Collecting 500 bananas and 25 golden bananas in each level, plus the two hidden banana fairies? Crazy).
Also my biggest achievement will probably be beating Jet Force Gemini 100% in my first playthrough (I'm in Tawfret with Juno and in Ichor Military Base with Vela), without any kind of help.
Also beating Paper Mario 100% will be a major achievement, being it so that it includes like 160 star pieces, 80 badges and 50 cooking recipes, plus a lot of other things!