My favourite 4*'s tend to be ones I use with my 5* teams:
Rigard – always loved using this guy, even more so now I have the costume.
Sabina – underrated as a healer
Tiburtus – def down is always useful
Wu Kong – but only for titans
Myst – the only S3 4* I regularly use, but I am missing most of them
Guardian Jackal – elemental def down for titans and mono stacks
Lady Wollerton – yellow healers are in short supply for me
Kiril – I rely a lot on 4* healers, plus he boosts defense AND attack
Grimm – see Tiburtus, my most-used hero until I maxed Isarnia
Sonya – dispell
Caedmon – a dispelling sniper
Melendor – a dispelling healer
Boldtusk – see Lady Wollerton but red
Gormek – another def downer, and a beefy tank for 4* tournaments
Scarlett – huge attack stat for titan tile damage
Guardian Falcon – elemental def down for titans and mono stacks
Wilbur – mainly for titans and tournaments
For raids I tend to favour snipers and healers, with the odd support hero thrown in.
There are others I sometimes use in tournaments such as Little John, Colen, Boril, Skittleskull, Hu Tao …
I don’t have Merlin or Gretel but they can be pretty nasty to fight against.
For 3*, max Kvasir. Those bees … grrrr.