My favorite system is probably the Game Boy Advance. It was one of the first systems I ever owned. Plus, the original model fits sooooo perfectly in my hands!
Other than nostalgia, it also had a lot of ports/remakes of older SNES, NES, and even a few Sega and Playstation games! Most of these ports can be argued to be worse than their original counterparts. But dispite a couple restrictions and compromises due to the hardware they were on, I honestly think they play just as well as the originals! Plus, It's mainly how kids in my age were introduced to such classics like Yoshi's Island, Kirby's Adventure/Nightmare in Dream Land, and Super Mario Bros. 3 among others!
Another thing is that the GBA was also portable. It would take a couple years for the SP to come out so I actually had a back light so I could see what I was doing, but still.XD I have fond memories of playing the GBA while riding in the car. I even had this awesome carrier case that looked like a giant GBA that could hold the system and multiple games!