Aromasin is IMO the best choice out of them all, however it is very expensive!
Arimdex and femara are both very effective estrogen inhibitors as well and are much cheaper then aromasin! Both arimidex/femara will have a negative effect on your lipid profile/cholesterol while aromasin appears not to have any negative effect!
Arimidex is weaker then both femara and aromasin and I beleive femara to be slightly stronger then aromasin when it comes to eliminating estrogen!
IF cost were not an issue then aromasin all the way!
IF cost is an issue then femara or arimdex are both good choices!
I think what choice you make will depend alot on what gear you are running and at what dosage! Remeber that completly eliminating estrogen or having even extrememly low estrogen is not good! So if you are running a very low aromatizing cycle then I think arimidex would be a good choice!
For guys like me, who are very prone to estrogen mediated effects, I use femara and always have nolvadex on hand for gyno!
Hope this helped!