Obviously post-1900, pick any country's Army, Air Force, Naval or Marine weapon that despite doing their fair share to win a conflict, it never got the recognition or praise they deserved.
I'll start the ball rolling with these few.
Army,
British Army's 25 pdr gun. Always overlooked due to the sometimes mythical properties of the German's Pak 88 which became more famous.
Air.
HAWKER HURRICANE. Always over shadowed by it's more elegant sibling the Spitfire.
BRISTOL BEAUFIGHTER. While the "Mossie" got all the plaudits flying and looking like a ballerina, the Beaufighter looked like it meant business, if it was a man, he'd be the bouncer on the door at your local night club and didn't mind getting it's hands dirty.
HANDLEY-PAGE HALIFAX. If the Lanc looked and acted like a professional boxer, the Halifax looked like a bar-room brawler who'd hit you to the floor then pick up a stool and hit you over the head with it. Again being over shadowed by the famous Lanc and it's missions.
Over to you folks.
Regards