Pat Riley -- My Car Is Straight GANGSTER!!

Screw the Lambo ... forget the Bentley ... NBA legend Pat Riley cruises around L.A. in one of the most GANGSTER vehicles of all-time -- and TMZ Sports has the footage.

The President of the Miami Heat was leaving a car wash in Malibu early Tuesday -- where only the finest detailers were allowed to work on his prized 1950 Mercury coupe.

FYI -- Riley has been OBSESSED with the 1950 Merc for years ... for good reason -- GQ once touted the ride as one of the most gangster cars of all time.

Check out the clip -- we also asked Pat what he got Dwyane Wade for his wedding ... and judging from Pat's reply, he hooked D-Wade up!!!