I should probably say a significant border with Russia. Norway's border with Russia for example is a narrow sliver of mountains. It's completely insignificant. Finland isn't part of this because they're not a NATO member. The Baltic states (Latvia and Estonia; Lithuania doesn't obrder Russia) are a little more nuanced. I imagine that was a tough pill for Russia to swallow but again the borders are small. Poland and Kaliningrad is also more of a technaclity.
This map [1] puts the size of the borders in perspective and also why Russia has made Belarus effectively a client state. And also why Georgia is in a similar position as a buffer between Russia and Turkey.
But Ukraine is of particular strategic importance to Russia not only because of the expansive border but because Ukraine is relatively flat. Here's a map of the Operation Barbarossa invasion route [2]. It was largely through what is now Belarus to the north and Ukraine to the south.
I can't find a similar map but I believe Napoleon followed a similar route.
While niether of these two campaigns were successful, quite famously, it's merely a function of geography.
Additionally, Ukraine's position is even more significant because it potentially threaten's Russia's access to the Black Sea and the occupited territory of Crimea.
[1]: https://www.nationsonline.org/maps/countries_europe_map-L.jp...
[2]: https://upload.wikimedia.org/wikipedia/commons/1/11/Marcks_P...