- Hungarian & Basque
I rank both Hungarian and Basque as Número Uno for two similar but qualitatively very disparate reasons: Their grammars and lexica are completely alien to Indo-European minds — Hungarian f*cks with your mind in ways you never thought possible, whereas Basque will make your run for the edge of the nearest cliff for one thing, if nothing else: the way it forms compound auxiliary verbs. E.g., “We gave them the book” must be rendered (and I’m simplifying here): “give + books + have AUX (obj. sing. + aux-pl.-to-pl. + subj. pl.).” It may sound easy, but a leap off a cliff will loom ominously on your horizon if you think it is [Approach with caution!] . . .
Finnish is just as alien as Hungarian in ways that are similar to Hungarian, just more manageable — 16 productive cases vs. 24, two alternatives for case for vowel harmony vs. up to four, plus (and this one’s a biggie) Finnish lacks the definite vs. indefinite verb conjugation that can be very challenging, to put it mildly.
Estonian is Finnish on Quaaludes — the case system is radically reduced in structure (e.g., Finnish: “in Stockholm” [“Stockholm” = “Tukholma”] = “Tukholmassa” but ”in Helsinki” = “Helsingissä” — Estonian: “Tukholmas” and “Helsingis”), and vowel harmony is non-existent, but it does have a less transparent morphology and a more complex phonology (just to keep things interesting). Hungarian also possesses vowel harmony, but there are three levels of distinction (o/e/ő).
Sámi is a Finno-Ugric language, along with Finnish and Estonian. From what I gather however, its grammar is less than half as complex as Estonian, with only six to eight noun cases. If one were to eliminate contenders in terms of something as superficial (in the literal meaning of the word) as number of noun declensions, then I would award that title to:
Russian. Because it’s tough, primarily due to the unpredictability of where stress is placed on verbs and nouns as they conjugate/decline — if you ever study Russian, it will drive you mad. Imperfective/perfective verb conjugations are most challenging due to the fact that: a) they don’t exist in non-Slavic languages and b) their formation is wildly unpredictable. Add to that Russian’s rules governing allophony, and you’ve got a lot of work ahead of you . . .