Cardiff is the best stadium by a country mile. Great site lines, good size, proper retractable roof, good atmosphere, good location, not too expensive for seats.
Old Wembley had the history and I have to admit, for night games the atmosphere was amazing when you got the electric crackle in the air bouncing off the roof. However, seats were terrible, terracing was shallow in the bottom tier. If you stood near the front of the upper terracing, your feet would be soaked in piss (Wembley waterfall was well known). Toilets and concourse were a joke, as was the underground and roads around it.
New Wembley considering it cost £850m is a poor stadium for the average fan. Of course the sight lines, seats and legroom is good, that should be standard for a new stadium of that size. What surprised me is that the concourses are still over-crowded and the toilets not much better than the old Wembley. The Arch is iconic, but useless and the cost of it meant they didn't put a proper retractable roof on it. It's utterly pointless having that bit of the roof over the goal ends that just slides back a little bit. I'm sure the suites and boxes are lovely, as are the FA offices etc, but for me, it was a wasted opportunity. The Dallas Cowboys stadium cost around the same that eclipses Wembley for facilities for all fans.