Think of the mentality involved in such debates, then extrapolate that to the type of people who would be passionate about it. Exceptions abound, of course, but in many ways it's a nerdier version of the WWE, with ego-driven hubris overshadowing inherently subjective comparisons and stories that were never meant to settle "who's best," and would in fact lose viewership if such distinctions ever become clear-cut. Think of the week-to-week bumps or knocks characters take in the public eye (in the Ownage thread, for example), and the fans who revel in the feats, then compare it to the momentary euphoria of a wrestler winning one of the belts before inevitably eventually losing it. Hell, we even had a "belt" system for the tiers over in the BZ forum for a while, where challenges for belts could be made. Exact same principle.
But - also like the WWE - it's often just fun, and has the anonymity of the internet. So it's funny to just **** around, or say things you don't mean, or clamor to the top of whatever hill your favorite is on, then trying to cling desperately to the top against detractors. Or stubbornly chill at the bottom with put-upon characters.
Anyway, on topic, Doomsday in DoS before the Superman fight has already been mentioned, but that one punch that sends Supergirl into gelatinous putty was always my favorite "holy sh*t!!" moment from it.