I just soloed with Russia without building a fleet (
gameID=185108) which makes me think that Russia would have the easiest time with it. They already start with two, and have access to the entire land interior of the board without needing to contend with Switzerland.
We all know that it's possible to do it with Austria, given PW's famous solo in the 2015 ODC (
gameID=159213). Austria seems more difficult, however. They have the same centres readily available in the interior as Russia. However, since Turkey is difficult to invade from the the Mediterranean or by land, there are less centres easily available.
Has anyone else soloed in this way with a different power?