If you need healing, then go with Vivica. Her heal is slow, yes, but I honestly think that complaint is overblown most of the time. I use Kunchen as my primary healer and I don’t notice much of difference in terms of having enough time to build his special before anyone dies as compared to when I was using an average Mana healer. She’s tanky enough to live until her heal is ready, too, at least in Platinum and low Diamond. Healers are useful in every situation, whether it be Titans, raiding, AW, events, etc, so she’d be the most versatile option.
Poseidon is good in most situations too, and there are a dearth of holy snipers (especially outside of 5*), so he’d be a great investment if you are ok on healers in general.
Rana is one of the better heroes on attack in the game, in my opinion. This is especially true in AW with field aid, where she can block the heals to 3 heroes. Her special is devastating in the right situation. It’s not as good when the game’s AI is using it and she’s not as effective in general in most events or stages where monsters can’t heal. She also isn’t very effective against Titans.
Ranvir is a useful hero, but since you have Wu, you should level him for now and save your darts for one of the other 3 heroes.
Just as a side note, Hansel is awesome and one of my favorite 4* heroes in the entire game, but he probably shouldn’t be your tank. Vivica could be if you max her, but I’d suggest using Rigard or Gormek in that spot until you level someone else to take over.