Not an easy decision between Tergat and Kipchoge, but I think I would go for Kipchoge, since Tergat never in has career was really dominant, which Kipchoge at the moment is. Also a 13 year span between global titles is unique.
But there are also other names which should be considered: Kipchoge Keino, David Rudisha, Moses Kiptanui, Ezekiel Kemboi... really not easy to decide.
Eliud Kipchoge should be in the top 10 all-time now, definitely ahead of Mo Farah. Much better PBs in the 3000m, 5000m, Marathon, also in the Half-Marathon. Almost equal in the 10000m. Farah's 4 doubles are also great, for sure, but his career record otherwise really is too weak. Without Kenenisa Bekele (arguably Nr. 2 distance runner ever, against he had to compete during Bekele's prime)), Kipchoge also would have more titles.
Isn't Bekele alongside Geb as arguably the No. 1 distance runner ever?