I'd say my favourite thing about Deep Purple is the interplay between Jon and Ritchie and the least favourite thing is the constant personnel changes that prevented them from building a dynasty like The Stones.
My favourite band is The Beatles. The best thing about them (in my opinion) is how they could blend their experimental tendencies with excellent songwriting and melody-making, something that few musicians managed to achieve. My least favourite thing about them is actually not their fault, but of their fans and of the musical critics, and is the fact that they tend to transform the band into some kind of be-all-end-all. I mean, these people made us believe that the Beatles were perfect and unbeatable in every possible aspect. As a person who was a fan of many different bands before liking Beatles (yes, it took me like eight years to become a fan of the band), I can say that they were beaten in several aspects. Lyrics, live performance, instrumental chops, form and substance, etc. As I said, I consider them my favourite band, but they were far from being perfect. If they were actually perfect, it could had made no sense in creating new music. That's my general opinion about them.