From a hardware sales perspective GBA selling so many units while being replaced so quickly was very impressive. Then again it was also cheap af. It was only $100/£89 at launch. Cheap enough that many families could buy each child their own as me and my brother got.

PSP was $250/£180, and Sony's first attempt at a handheld.

The thread is really asking which system is 2nd place.

all 3 of these systems were fantastic.... its honestly a tough question.

I prefer the PSP & 3DS above the GBA.
But between the PSP and 3DS?....  

Edit 2 : See my post here.

GBA was the fastest selling in USA after 9 months on sale.

Right .. well we don't have monthly US data for it, to do a proper comparison .. Switch 2 has a chance of beating it if it continues to do well enough there. The data I managed to find is 6M+ by June 2002 (first full year, Switch 2 is likely already there with May), so they are on par. And then 12M by January 2003 and 20M by February 2004.

Probably the GBA, but an argument could be made in favour of the PSP - Sony had the added challenge of breaking into a market Nintendo had dominated for years. They went from 0% market share to 34.9% despite competing against the original DS.

That's true. The GBA basically had no competition (wonderswan color and later ngage) while the PSP had very strong competition.

Based on data I found, official from Nintendo, alongside US table which I've got from this very site years ago (I am not sure from who exactly), I made estimated chart line for the GBA US sales, and added the Switch 2, to see if Switch 2 can beat the " fastest selling system " in the US. All data is pure sales, since the reports and data from Nintendo about the GBA, are units sold. Switch 2 sold data we have from here - VGChartz. The official data is from random points in time, therefore with that and the help of the table below which is for calendar year sales, I did the chart line, all holiday points are accurate, but the period between them can't be all 100% accurate cuz we simply don't have data for each one, only for some. So where data is available, I put it right there, where it's not I even it out. But in the end it should be close enough.

Official data from Nintendo, (there is sources for each one, despite it's from GPT):

They were different gens so... none? lol