Alright so assuming you can max everything you got:
My advice for a fairly uniform defence would be:
Natalya, Ariel, Seshat, Neith, Morgan Le Fay.
You don’t really have a fantastic tank so I kinda went with what gives a big damage output in Seshat & who gets the boost from the minions.
Natalya & Morgan are IMO some of the best wingers around. Both are fast speed & have a humongous DoT output. Ariel is a great buffer & Nieth is probably the best of your holy bunch.
Red - Fire
My focus would be on Wilbur, Falcon & Boldtusk. These three will get you through a HUGE amount of events, challenges, raids etc…
After that, I’d look at Natalya or Marjana for use on the defence team in a wing.
Green - Nature
Melendor is a very handy healer. His big attack attack stat means he’s useful in more than just a healer role.
Little John is also a handy one to have. He’s got a bigger attack stat again & the mana slow can be painful… problem is he’s slow
Morgan le Fay is my pick of the 5* because I think she’s quite possibly the best winger in the game… I also love raiding with her & have seen her solo kill an enemy team.
Blue - Ice
Kiril & Grimm & Triton are all solid 4* heroes. They are staples on many people teams. I personally still use Grimm in raids & titans with +15 emblems
Ariel is definily the pick of the 5* here. Reasons are obvious but main one being that shes an Average Speed heal all, cleanse all & mana buffer
Yellow - Holy
Li Xiu is probably the best of the 4* here
Wu Kong should also be a priority. He should definitly be a staple on your titan teams for the big attack buff that he gives. See below for a link to an analysis that I did comparing the various titan buffers.
Of the 5* you have, Nieth is probably the more versatile. She’s not great but I don’t think you’ll need another 5* healer, especially a slow one, as you will likely focus on Ariel.
Purple - Dark
Rigard & Proteus would be my picks here. Proteus’s mana blocking is on point. Rigard is a fantastic healer & cleanser.
Merlin & Tibs are also great & would probably focus on them ahead of Dom & Obakan
Seshat is easily the best of the 5* dark heroes you got available. Her resist mana block makes her immune to Guin, Mitsuko and heaps of other heroes (quite a versatile one). Also the minions generate themselves, giving her probably the best meat shields around.