Depends how you measure language complexity.
Some languages have hard pronunciation (vietnamese, chinese, arabic)
Some have hard writing system (arabic, chinese)
Some have inconsistent spelling (English, French)
Some have complex grammar where you need to know a lot of things just to form simple sentences (Finnish, Hungarian)
Of slavic languages, Russian is definitely not the hardest. Czech language has more grammatical cases (7 instead of russian's 6). Polish has 5 grammatical genders (russian has 3, just like german. Romance languages (spanish, italian, french) have just two)
Also people saying Finnish is the hardest language might be wrong. Sure, the grammar is difficult (15 official grammatical cases but there are actually more). You need to know a lot more than to just put words together (like in English). But the writing system, spelling, pronunciation are all extremely easy. If you know how to spell a word, you know how to pronounce it. If you know how to pronounce a word, you always know how to spell it.