DS hands down; GBA comes a close second though. I mainly like the DS because I had an extensive game library, and the second screen touch screen gimmick was so fun. A few games where I loved utilizing the touch screen were the spin-off Pokemon Ranger series, Cooking Mama, Warioware: Touched, Pokemon Dash, and Mario Party DS.
GBA has this really fun library and games were relatively not as expensive back then (or at least, from what I remember) so it definitely comes a close second to DS. GBA brings me so much nostagia of my childhood, and until now I still find myself enjoying some GBA games. I love the pixel-y style of the GBA era, and the gameplay of a lot of games still has me hooked to this console.
While I consider it maybe 4th (with Switch being my 3rd) honestly, it's a shame I didn't enjoy the 3DS era as much as I did the DS era. At least for Pokemon, the series lost its charm for me once they started catering the graphics towards a less pixel-y and more 3D style. I never liked or even used the 3D gimmick of the 3DS; it just gave me headaches and it felt like a waste. I did enjoy Mario Kart 7 and New Leaf, very revolutionary games, but a combination of growing up and not being able to afford games anymore made my experience with the 3DS era less fun compared to DS. (Also, I'm very sad that they didn't make any more Pokemon Ranger games on the 3DS, that series was definitely 100% underrated.)