Finland sort of looks like a person and so my unfairly unpopular town is called by some "the bottom of Finland," usually with cruder language.
I grew up in "Athens of Finland" (for culture and education) and lived near "Chicago of Finland" (people assume it was because of crime rates but actually was because of the rate population grew or something).
Well, I live in the traditional rival city, then - the Manchester of Finland, the Brick City, the Red City, Nääsville (1).
I spent good chunks of my childhood in the town that steals the j's (2), or around the World's Navel, the Kalakukko City (3).
The town I grew up in (Hämeenlinna) hasn't a nickname as a whole, but it's centre is called Finland's largest lit cemetery.
(1): the city of Tampere. The centre of Nordic heavy industry in the 18th and 19th centuries, lots of red brick and huge old factories - thus the Manchester of Finland, shortened Manse. In the 1970's and 1980's the Finnish rock and punk scene coiled around the city and the nickname was further justified. The Brick City is simply because of the vast amount of red brick. The Red City has to do with red brick as well, but it also stands for the Red Declaration (for workers' rights) read there in 1905, and the city being the headquarters of the Reds, the defeated side in the Finnish civil war. Nääsville is a play on Nashville, because of the constant occurrence of the phrase "nääs" ("you see") in local speech which sounds just like a prolonged "Nash" with an ordinary s.
(2): Kajaani, which would be pronounced similar to "Kah-yaa-knee", but the dialect spoken there stresses the "yaa" part, so it's regularly transcripted as "Kajjaani", sometimes with three j's. Thus, no other cities in the northern parts of Finland may be pronounced with an additional "j" (some dialects tend to do Joensuu - Jojensuu), as Kajjaani steals all the j's in the north.
(3): Kuopio, the capital of Upper Savonia region. Locals ironically, and just to frustrate non-Savonians, call it the World's Navel - it's relatively big for a Finnish city, and located in what used to be the middle of nowhere, and still is to some extent. The kalakukko is a traditional Savonian dish (little fish and bacon baked inside a rye pouch), and both of the most remarkable bakeries providing those are located in Kuopio.
It's kinda funny, though, that no more nicknames for Kuopio exist, since it's deep in the Savonian culture to never refer to things with their actual names. Countless euphemisms and nicknames are invented instead - sometimes to the point of losing the original name. For example, there is a monument in the city that has two flattish, narrow metal slabs pointing upwards, connected at their lower ends. The actual name of the monument is a complete mystery, everybody has called it The Frozen Longjohns for decades now.