As far as advertising I'd have to say SNES vs Genesis but that is probably based on my age as well: My family had a Coleco system and a NES, but I was probably around 7-10 years old and I didn't take notice of console fanboyism.
The first time I noticed an actual rift between players was PS2/GC/XBox largely due to the internet and gaming forums. I never understood and to this day don't understand the point of fanboyism and hoping that one console will fail in the face of another. In my opinion, competition is good for the gaming market.
As far as my own fanboyism/bias I don't behold myself to a certain company indefinitely. I definitely favor one console over another in a given generation and hope that that console gets as many good games as possible, but it is not based on brand, it is based on what the company is doing in relation to my interest. For example, I swore off Microsoft because the XBox just didn't deliver for me, and I was pissed off when I bought it and then like a year later they announced the XBox 360. I think I own 5 XBox games in total. But now the XBox 360 is the only current gen console that I own and I am quite happy to have it stay that way until either Sony gets their act together or until the next generation rolls around in ~2010 (which is the earliest date I would even consider buying into the next generation of any console).
