Do you have any other heroes maxed? If so, who? Hard to give advice with only a little bit of info
Without knowing more, I’m gonna go with:
Zeline
Her dispel is invaluable [offensively], and her attack down makes her great vs blue titans. Also, she makes a great flank defensively. & With her fast speed and SS, she’s the most versatile of the bunch. So if you don’t have many 5* maxed, she is gonna be the safest option [for you]
Edit:
After Zeline, the choice is definitely more difficult; imo all of your options are viable. Really would depend on who else you have in your roster, and what you need most
Ratatoskr
If you lack a solid 5* healer, and wanna up your blue titans damage. Although, this one benefits the least from seeing 80
The Hatter
Mostly for fun; doesn’t play well with Zeline or other dispellers, and being average, he’s usually behind em all too. Although, he’s a blast during VF, or if you have a level 23 green mana troop
Lady Locke
Great for her all team cleanse, as that’s pretty rare, and her DOT is vicious. Not great defensively tho, but her tile damage would be useful against titans and in stacks
Atomos
Could make a pretty solid tank, depending on your other options [there]. Better defensive hero (imo), but offensively he’s nice for the AoE hit (assuming you’re not using/stacking hit 3’s like Evelyn/Almur and Buddy/Marcel - as they would do better with someone like Hatter)
Elradir
I’m really not a fan of this one, but his elemental link is quite impressive, so if you run mono or a deep stack, he could be useful for that alone. However, he’s also a solid counter to DOT heroes, if you’re struggling with those I spose (but works better with 2+ of him)