You’ve already received good advice. I will add a bit, though I don’t differ much from what is below.
@Aquaginera_7DD raises a good point about Sonya’s debuff. Since the system will fire your heroes’ specials left-to-right, you want Sonya to cleanse your team first. (Note that without the costume, she debuffs your enemies, which is an even better reason to put her on the left.)
Several folks suggested you put Jean-François next to Telluria. If it’s not clear, that’s because as your opponent stacks red against your tank, those red tiles will charge your flank.
I have Freya, but am still leveling her, so I don’t have much experience with her. Still, since she’s at 3.70, I would go with her.
I am a fan of Poseidon, and have him in my defense team. I just drew Malosi, and read good things about him, but no direct experience. Vivica is slow, and while a 5-star hero should be on every roster, she would not be my first yellow five-star to max. Still, right now, she’s you’re only bet.
Ergo: Sonya - Jean-François - Telluria - Freya - Vivica.
I’d swap Poseidon (or Malosi) in for Vivica as soon as you can get one of them leveled up.
Lastly, there are resources such as Titan Mafia that will show you frequency in which a given hero appears in which position. Right now, per that source, Telluria is the most popular tank, but also, when she appears in a defense lineup, she almost always appears as a tank. Seeing what others do with their heroes is an interesting starting point in how I think about mine.