I have one full day in Madrid in late May and have never been there before. I know it is impossible to see/do everything so I would like to know in your opinion what are the must see sites, neighborhoods, museum. What's the best area for a Tapas Crawl?

Please help

definitely take the hop on /off bus. you can end up near the plaza mayor and go into one of the cuevas (cave bars) to get some atmosphere and some tapas.

then i would try to go to a flamenco show, and stroll towards plaza oriente, and have a coffee or drink as you soak up the view of the lit up royal palace.

also try to take the tourist bus once the sun has gone down so you can see the city with the lights and well illuminated buildings..
the central plazas on the main road " la castellana" are always a joy to see over and over.

Prado
Royal Palace
Plaza Mayor

(We never found much more than this to do and Madrid is the only city in europe that we have ever left early - a late April heat wave in the upper 90's didn't help.)

Center Reina Sofia is a wonderful modern-art museum. I didn't get to the Thyssen-Bornemisza art museum, but very much want to do so. The Prado, of course, is excellent. Walk through the Lavapies district.

Hola

Madrid is a beautiful city. There is an incredible number of things to do. I agree the hop on /off bus (Marid Vision) is a good way of getting the gist of the city without wearing yourself out. If you're there in the evening I preferred the Plaza Santa Ana to Plaza Mayor. Stroll down Gran Via to view some gorgeous architecture.