Isarnia is great, and against titans she’s useful… If you have a heavily emblemed Grimm and don’t intend to emblem her, then don’t do her for titans alone.
Having said that, I enjoy her in raid stacks and events too.
Vela is interesting… She’s not a top tier HOTM, IMHO, but she’s still a good hero.
The +crit will help you in stacks (so titans at very least) and the -attack to fire enemies could be gold on high level titans!
Richard is a decent tank… If you need a war tank and/or have a lot of Paladin emblems burning a hole in your pocket then you may find a use for him.
Thorne likewise is a sturdy Paladin. Special does more damage than Richard, but there’s no secondary effect - he’s underestimated, but nothing special (not awful, just “meh”).
Should be between Vela and Isarnia IMHO.
What’s the rest of your deck like?
If you have hefty AoE in other colours and have Grimm maxed (preferably emblemed), I’d probably lean towards Vela personally… But it’s kinda down to how you play and what you like using.