For me it would be 14.1.
Why?
First of all, it is the game's best teacher. You can challenge yourself constantly with 14.1 - and you will also learn intricate position, patterns, breaking up clusters, etc.
Playing a good game of straight pool requires excellent fundamentals. Your game relies on perfect application of the basic fundamentals in the same way that a chain relies upon its links. Weaken one link and the chain is useless.
I wish I had a dime for every time a player asked me what they had to do to get to the next level. I usually tell them to learn how to play straight pool.
The next time I see them, they are usually playing 9 ball.
For me, I practice 14.1 because it keeps me from getting lazy.
I can constantly challenge myself. During practice, I can constantly and very easily apply situational pressure that is equal to the pressure that I will experience during competition. 14.1 demands my best performance at all times, and it very rarely leaves me feeling as if I was not challenged.
I talk to a lot of players that just play 9 ball. They feel as if something is missing from their game. I believe that what is missing from their game is a firm foundation based in 14.1 continuous.
FWIW, when Johnny Archer practices, straight pool is a LARGE part of what he works on every day - he plays straight pool more than 9 ball.
Look at some of the great all-around players - Danny Harriman, Oliver Ortmann, John Schmidt, Thorsten Hohmann, Johnny Archer, Max Eberle, Niels Feijen, Mika Immonen, Allen Hopkins, Ralf Souquet, Nick Varner, Mike Sigel, Grady Mathews, Ray Martin, (the list goes on and on) these players all have a solid foundation to their games based in straight pool.
Warning: Ralf Souquet lost all of his hair by playing too much straight pool. Back in the day, every time he missed a shot he would go into a rage and pull his hair out in clumps. Today he is bald not because he wants to be stylish; it is to keep him from hurting himself. We have no idea what happened to Forsyth's hair.