It was between Mega Microgame$ and Gold for me, but Mega Microgame$ wins out for its aesthetic. The Mega Game$ era in general was amazing aesthetically, it looked rough and scrappy like it should, the cutscenes were a little surreal and pretty wild and all the music had this nice distinct vibe to it and the sound design was great and the UI in the GBA one had the computer thing going on and...it was just great! I think the designs took until Twisted! to peak though
And yet in terms of gameplay, WarioWare Gold was the peak. Aesthetically the series had become diluted by this point and the new voices and visual direction depress me but I can't deny how awesome the gameplay loop was in that one with all the different microgames in many of the old control schemes re-touched, mashed together and twisted into a tidy little package