Best "Rhapsody In Blue"?

The Tilson Thomas is very very good.

Also exceptionally good is:
Slatkin/StLouisSO, Siegel -pf
(this is the performance I play over and over again)

Also good, but somewhat limited by the orchestra's ability to swing...
Previn/LSO, Previn -pf
Andrew Litton/RPO

The Tilson Thomas is indeed one of my favorite. Great sound but more emphasis on the Jazz band, it seemed somewhat off to me.
Bernstein's is tremendous, great sound, great dynamics, great playing.
Would add Lincoln Mayorga's on A Gershwin Celebration CD on Sheffield Labs, probably my overall favorite. It has a quicker pace than Bernstein's but slower than Tilson-Thomas'.
It also has a Russian orchestra that may or may not matter.

Bernstein does an excellent job of bringing together both the jazz and classical idioms, and he is given very fine sound to boot.

I believe you should consider Michael Camilo's recording with the Barcelona Symphony; it's available in SACD (and CD) on the Telarc label. That's my favorite! What a great piece of music!

Fiedler directing the Boston Pops with Earl Wild on piano (RCA Living Stereo - SACD/CD hybrid) is an honest performance with very good sonics.

no one interprets gershwin like oscar levant did. start there for sure.