I've come to the same conclusion about the H-19. It's tempting to say use by all the services
in any significant numbers.
It's one of my favorite types...and the a.c. I'd most like to fly in. It was the first helicopter type I saw in person. The sight and smells...(oil, fuel, hydraulic fluid and canvas) of that Rescue H-19 at the AF base I was born at, remain with me almost 60 years later.
Flying Illini beat me to the more obvious answer...though USMC operations with the Blackhawk are rather sparse. However, if we count White House use as full ops, (and not just an asterisk, since no average Marine has flown in one), then we'll have to add the Sikorsky H-3 series to the mix....USN SH-3s, USAF & USGC HH-3s (based on the S-61R) and finally USA & USMC use of VIP VH-3s. Though almost forgotten today, the Army shared White House transport duties in the early 60s, if you look at JFK-era photos, some of the VH-3s are Army ships.
And like the USMC Blackhawks, the Marine use of the Sea Kings is limited to VIP use.
Likewise, the H-34 (S-58) saw some use with all services...in addition to the well known Army, USN and Marine use, a few (6) were used by the USCG (who lost two in one day to engine failures during the search for a B-47 crew in Tampa Bay, Florida) and the USAFR had a dozen or two ex-Navy aircraft equipping rescue units in the early 70s....before they got HH-3s when they were replaced by HH-53s in active duty units.
Getting away from Sikorskys, you could add Bell 47/H-13s to the list...I've seen some in USMC and AF markings...though the AF didn't have many. Aside from a few E's and H/G's used for local base rescue in the 1950s, the most famous AF ones are the two "J" Rangers assigned to the White House circa 1957.
C-45s also count since the CG had a couple, likewise T-6 (counting the USAAF use on WWII as Army).
Finally, the C-47....but that's almost a given.
