What's The Strongest Base Strategy in Thurn & Taxis?
In my experience, this is exactly the fun of the game and all its expansions (and a few others like it): there is not one strategy that is guaranteed to do well, but you have to constantly adapt your strategy to what cards are available, what the other players do, etc. I would consider this a virtue of the game, and would typically consider a game that has one winning strategy to be broken.

It often happens to me that I play a game that's new to me, lose to someone more experienced who did better in one particular area, and decide that improving that area is going to be my strategy because I now see that it is 'obviously' better than concentrating on other areas. Then in the rematch, the positions are reversed and the other player creams me with my 'original' strategy. The lesson is that the difficulty of the game is not in implementing a strategy - anyone can do that - but in recognizing which of the many available strategies is called for in a given situation.