The "best" AI

Tamoxifen isn't an AI—it's a SERM, which blocks the tumor cells’ access to estrogen but doesn't prevent its synthesis, so it doesn't even belong in this discussion.

As to which AI is “best," different MOs have different standards as to which one they will initially prescribe. Letrozole is the third-generation AI, and some studies say it's a bit more effective than the others. Exemestane is the only one that is steroidal. Anastrazole was the first on the market and is still initially prescribed more often for thinner women.

I say “initially prescribed" because different women react differently to different AIs and even to different mfrs.' generic versions (“brands" in the general sense, as opposed to the original patented brand) of a given AI. Many women have to change AIs because of degree & type of side effects and many elect to try different brands of the same generic. The goal is to make compliance with the AI regimen last as long as possible, and if that means switching generics and trying a different AI, so be it. Whatever it takes to get the patient over the goal line without quitting.