From a
Russian perspective...
I vote
Pandemia, say if the H5N1 virus mutates into something threatning to humans like in 1918.
Otherwise...
Terrorism
Negligible, and anyone who says different, practically anywhere, has been brainwashed by the government. Probably more people die from car accidents every year than have ever been killed by terrorism.
Economic downturn / Unemployment
Considering the economy is booming, there's a double surplus and 0.5tn $ in FOREX reserves, I'd say - not a threat.
Immigration
Nope.
Global warming
It will only benefit Russia. Otherwise, that's what I'd choose if I was in just about any country outside Russia or Canada.
Alien invation
No comment.
Poor education
Russia is relatively well-educated
War
Perhaps a distant second to pandemia, but I don't really envision anything big (ie with NATO or China) blowing over any time soon.