I know I'm going to get flamed, but I'll lay it out for all to comment on. I don't get all the hype on delis.
I've had Brent's, Art's, Weiler's, Billy's and Solly's. While I like most of them, nothing really stood out as the "best". What am I missing? I usually go with the half pastrami or corned beef sandwich with a bowl of matzo ball soup.
My personal preference is to just go to wherever is closest to where I am at any given time. It's a deli, and it probably will be good wherever you go.
I have to say, that I prefer the matzo ball soup at Solly's the best. There's something in the matzo ball that makes it taste better than the rest.