Reuben, hands down...but I had no idea I was in such good company!
Actually, let me qualify that - my reubens are corned beef only (no pastrami).

And the recent poster with alfalfa sprouts, avocado and tomato reminded me of what I lived on while in university...but with mustard, rather than mayo.

I didn't eat red meat for about 12 years -- by choice -- but like Punki, made an exception for a reuben about once or twice a year. Now I eat meat...but live in a country where I can't find reubens, much less corned beef... so really miss out!