Can't remember that much since its been a while, but when "Kaiba" made a cameo and summoned himself.
You mean Kaibaman? I remember being excited about that episode, but it turned out to be pretty generic.
I really loved the Society of Light arc. Sartorious was an awesome character, and the whole thing was really ironic because light is usually portrayed as good, not evil
I hated that arc to be completely honest. I never found Sartorius to be interesting and I hated the whole Neo Spacian stuff. I'll admit that I still watched that arc despite not liking it much, but that was just because I was really hooked on the anime at the time. My take on the seasons: Season 4 > Season 1 > Season 3 > Season 2.