which is the best version

[Grumbler]

Isn't squad battle by Microsucks? Doesn't that answer the question?
As far as the different flavors of SP, you can only find out by playing them yourself. I play MBT because I am a 'what if' guy and the WW2 era holds very little 'what if's', having been chewed over like Tom Hanks running shoes in 'Hooch'.
MBT is also the most complex of the SP series. Lately I have been helping Combat Mission players convert/learn MBT. Bringing them over to the dark side, so to speak.
An important part of all SP games is managing the firefight. This is a key difference between SP and most war games. Controlling which units shoot at which targets and in what order really helps in winning firefights. It's not enough to win the game by itself, since winning a fire fight that doesn't move you toward your main goal is a waste of resources. Anyway, I have noticed that the CM guys are good at managing the firefights, which surprised me.
If you are looking for suggestions, try SPWW2 first. playing small battles against human opponents. That will get you used to the mechanics of the system and a gentle introduction to playing against Humans.
Get a couple games under your belt, then expand the size of your WW2 battles so as to get an idea of how the difference in scope changes your tactics. You will also have to learn how to plan and the importance of reserves.
Then you will be ready to give MBT a shot. If you have never had to bust a Pak front, then you will have no chance of solving the tactical problem presented by a dug-in platoon of ATGM's.
If you want to help out the rest of the blitz, track down Jadpanther and see if he will 'mentor' you at SPWW2. Jad is in a burnout phase, I think.
After leading the ladder for the last eon or two, he has gone semi-AWOL.
Most likely he is shacked up in Paris with some war widow, living off the blackmarket and making muster just often enough to keep the Mp's from tracking him down.
Maybe training a rookie would get his interest levels back up.