There are so many good Game Boy games, I can't see how that can be overlooked, although I wouldn't recommend anyone playing them on the original handheld. GBA SP 101 is the way to go for that (or if you want to shell out some cash, a front-lit GBC is more comfortable).
Still, I think the other two can be skipped without significant loss. The AES is stupidly expensive, even if you get an MVS converter. The NGCD is all right, but many of the games on the system can be had elsewhere, leaving about a dozen exclusives. I recommend it to folks, but it's not a must-have.
The Sega CD is a vastly overrated console among collectors. Maybe if you're a hardcore RPG fan, the Lunar Games, Vay, and Shining Forced CD might make an investment worth it, but Android Assault and Silpheed are not actually that good (that might be personal, though, as I cannot stand the "2.5 D" overhead perspective; give me a standard 2D shooter!). What else does it have that other consoles do not? It's mostly polished ports, giving us good but not way better versions of Final Fight, Earthworm Jim, and Batman Returns.
Oh, and Sonic CD is the worst of the 2D Sonic games (unless you count Knuckles Chaotix).