Of course Momos. We understand they are ubiquitous but authentic momos of SIkkim and Darjeeling beat the ones found in any other place. Dont forget to explore this shady little outlet below Metro near MG Road in Gangtok for Momos.
Apart from that there is chowmein, Shaphaley, Selroti, Thukpa, thenthuk, and lot others.
If you are looking at full meal, ask for Thakali Khana. It includes other special local items like, Kinema, Gundruk Soup, Ningro, Churpi, Sollar.
Not to Forget Darjeeling and Sikkim Tea.