As for the actual question, I generally prefer the old generation. However, that's not to say I don't like the new generation; not at all. There are quite a few screw-ups in the old gen, and lots of examples of great ones that have just come out. Just, on the whole, I like the old gens more. :/
It's not surprising that they are declining in quality, simply because there's less things they can do. Furthermore, there's a sense of nostalgia attached to the older gen pokemon.