My opinion is that the British Bomber is the most important for a number of reasons.
1) It is a very vital and functional piece of equipment for the British Empire. It has any number of uses in the first round of the game and gives range and punch to the British navy and army in subsequent rounds.
1a) It can be used to liberate Egypt and secure Africa
1b) It can be used to SBR Germany limiting their production on round 2
1c) It can be stationed in Novosibirsk to threaten the Japanese fleet almost forcing them to hold back some warships in SZ 60
1d) It can be used to sink the German Baltic Fleet
1e) It can be used to liberate India on UK 2
1f) It can be used to sink the Germans if they move their fleet west to Gibraltar
Etc.
2) It is very expensive to replace, especially for England.
2a) England normally loses Africa for a good portion of the game, and they lose Australia, India, Madagascar and New Zealand as well as possibly Persia and Trans-Jordan too. This means England’s hoping to earn in the mid to high teens. A bomber costs almost that entire pay check.
2b) Russian fighters only cost 10 IPC and Russia can expect to be earning in the mid to upper 20s for much of the game. (This explains why almost every game results in a Russian air force of 3 or 4 fighters before the end of the game.) Therefore it is significantly easier to replace Russian fighter losses than it is British Bombers.
2c) Americans are wealthy. Even if all their outlying territories are conquered, America is still earning 28 IPC meaning they can still build at least one bomber and have money left for navy or ground forces. But most likely, America’s earning in the upper 30s and can easily build two bombers a round if they so chose.
Therefore, I have determined, in my opinion, that the most important piece on the board is the British Bomber for two main reasons: 1) It is too expensive to replace given England’s finances. 2) It is too valuable in extending the power of the British empire and in slowing down Japanese and German expansion.
Note, this is just my opinion. If you, personally, feel that the Russian fighters are more important, then that is great! I am not going to force my opinion on you, your playing style is probably not the same as mine. After all, how many of you purchase 2 bombers on UK 1 and 2 Bombers on USA 1? Probably not many. And if you did, then your valuation of the British bomber may not be as high. Heck, if you routinely end up with England owning all of Africa and W. Europe and Norway you may feel the British Bomber is much more expendable than the Russian fighters (maybe even more than a Russian tank) as well.
I cannot read your mind and I do not have the drive to go investigate every game you’ve ever played - even if I had the ability. All I can say is, in my personal and humble opinion, the British bomber has been the most valuable unit for my strategies as the allies and, as the axis, I generally go out of my way to kill that bomber as fast and soon as possible.