As far as I know, d4 is best because it takes the center.
When I face a Caro Kann player, I usually play d4 because I like the resulting positions as white.
But I have also played Nc3 or d3 if I dont want to give Black his probably well known lines.
Nf3 and especially f4 look weird, because black gets the center or at least the initiative with d5.
2.f4 is usually used to keep Caro-Kann players out of their comfort zone. Speaking as a former Caro-Kann player, I can tell you that if you play 2...d5, you will usually end up in some weird kind of Advance Variation. White will sometimes play d3 instead of d4, but taking on d5 just puts a misplaced pawn on f4 in the Exchange Variation, and 3.Nc3 leads to issues for White as there are lines in the Classical Variation where f4 isn't very good, and does nothing but weaken White's position. If you are memorizing lines, then I applaud the 2.f4 player and give you 2 thumbs down as no opening should be studied via memorization. If you truly understand the ins and outs of the Caro-Kann, 2.f4 should come to you as a joke and you should have no trouble equalizing.
2.Nf3 is typically played by someone either looking to specifically play the Two Knights Variation (3.Nc3), or in most cases of lower rated players, they don't understand any other move because all they did was learn the basics of the Ruy Lopez, and think that all openings should follow the same script. These types of players tend to despise tension. They don't understand the concept of keeping pawn tension in the center. They don't get the concept of keeping a Knight pinned by a Bishop. They will be quick to trade. These players will usually play 3.exd5 after 2...d5, and this is not good for White at all. In the Exchange Variation, White should be delaying Nf3 as long as humanly possible, making it difficult for Black to develop his Light-Squared Bishop properly. If White doesn't play the Two Knights variation, and ESPECIALLY if White exchanges on move 3, 2.Nf3 should come to you as a complete joke, and having the ability to play an early ...Bg4 should be considered a complete luxury for you!
The Caro-Kann Defense is started with 1.e4 c6. I play the Caro-Kann a lot as black. Most strong players play 2.d4 d5 after that and this kind of thing happens:
The thing is, i'm currently at a level where almost NOBODY plays 2.d4. They usually play 2.Nf3 and less commonly 2.f4
I have no clue what the theory is or why these moves aren't played by GMs. Basically I know that 2.d4 is the best but I don't know why the others are not. So my question is this: Can you give me any insight as to why 2.d4 is the best and why the others are not (2.Nf3 2.f4 and any others that someone might play)?