A comment on the best-to-worst list: at the high end, Shimano groups are comparable. XTR comparable to Dura-Ace, and XT comparable to Ultegra.
At the low end, however, they're not comparable, because mountain groups go much lower in quality than do road groups. Why? Because there's demand for cheap mountain bikes. There's little-to-no demand for cheap road bikes (although there was in the 1980's, before mountain bikes became sexy and took over the cycling imaginations of most people, and thereby took over nearly all of the low-priced bike market). I'd say taht Sora is about equivalent to Alivio in quality (based on my intition and knowledge of the groups), which just so happens to be equal # of steps from the top if you count down from Dura-Ace and XTR.