It depends on your other gear. I personally like fortitude on my gf barb merc, faster ias. Helmet can be many things, crown of thieves for more gold, kira's guardian for no chill and big resists, guillaume's face for crushing blow. Whatever you have and want to test out. Then use the ias calculator to see what you need if you want to or need the extra ias, or maybe other things can be more useful. Eth rune isn't half bad, -25% enemy defense means more hits. You can use the level 23 enchant from demon limb if you want to bother with it, he will hit much more often.
FORT was a huge part of my success and a IAS DAM gem was a great idea.
I need cannot be frozen, Cham in Helm or ring my best option? or something else?