First, all 3 heroes are very good. Whichever you pick, you will not regret it.
I would pick Zimkitha for a lot of reasons. Fast cleanse, attack buff, hits everything. She is useful in farming, raids, titan battles, challenges, everything. Plus her Elemental Link gives a little healing to all reds. The one argument I see against her is that you have Rigard as your only heal-all, and he cleanses too. But in my opinion, you can’t have too many cleanses.
Grazul is nice, but offers no offensive benefits. She is 100% defensive, and her heal is only so-so. Also, her debuff resistance is tricky to time correctly. She is good, but I think Zim is a much better choice.
Ares is a very good hero. He is probably the best red to use as tank in raid defense. He’s also a fair healer and gives a strong attack buff. But he is not as versatile as Zim. He is a strong supporting hero, but can’t deal any damage himself.