The Merlin engine was used in over forty aircraft during World War Two but it is mostly associated with the Supermarine Spitfire, Hawker Hurricane, Avro Lancaster bomber and the de Havilland Mosquito.
Then there were the Mustang, Battle, Halifax, Wellington, Lincoln, Sea Hornet, etc etc. A Merlin was even put into a captured Messerschmitt Bf 109, all RAF personnel concerned agreeing that the aircraft was now much improved by the change. You might expect this kind of verdict, just as when a captured Spitfire was fitted with a German DB605 engine the same conclusion was reached by the Germans! Here is a full list:
Armstrong Whitworth Whitley
Avro Athena
Avro Lancaster
Avro Lancastrian
Avro Lincoln
Avro Manchester III
Avro Tudor
Avro York
Boulton Paul Balliol and Sea Balliol
Boulton Paul Defiant
Bristol Beaufighter II
CAC CA-18 Mark 23 Mustang
Canadair North Star
CASA 2.111B and D
Cierva Air Horse
de Havilland Mosquito
de Havilland Hornet
Fairey Barracuda
Fairey Battle
Fairey Fulmar
Fairey P.4/34
Fiat G.59
Handley Page Halifax
Handley Page Halton
Hawker Hart (Test bed)
Hawker Henley
Hawker Horsley (Test bed)
Hawker Hotspur
Hawker Hurricane and Sea Hurricane
Hispano Aviación HA-1112
I.Ae. 30 Ñancú
Miles M.20
North American Mustang Mk X
Renard R.38
Short Sturgeon
Supermarine Type 322
Supermarine Seafire
Supermarine Spitfire
Tsunami Racer
Vickers F.7/41
Vickers Wellington Mk II and Mk VI
Vickers Windsor
Westland Welkin