I have three contenders, the early Claud 4-4-0s, the E4 2-4-0s and the J65-69 series of 0-6-0 tank engine.
For me, the best-looking versions of the Clauds and the best performing were those rebuilt under the Gresley/Thompson regime so I for the purposes of the poll, I am afraid they have to go.
The E4 2-4-0s were a very versatile and long-lived class, some lasting more than 60 years, and the last 2-4-0s to remain in BR service. The J65-69 tank engines were also very versatile, performing well on a range of duties from shunting to the Liverpool Street suburban service. They were also long-lived with examples lasting over 60 years. However, they were probably more widely dispersed across the LNER network (I have a 3D printed example numbered 7365 which spent some time at Hitchin). For that reason, they form my nomination.