4* heroes take 47% of XP and half the 3* ascension items of a 5* hero, so you want a good team of 4* first. Preferable two good 4* rainbow teams so you can use double strong colors against a titan and remove your weak color from you attack team.
Stronghold 20 and legendary training will give you 4* heroes ( Note 0 )
Rigard is very nice ( Note 1) and helps you complete Rare Ascension Item Quests to help you level other heros.
Melendor/ Sabina are almost clones, but since you already have Rigard, I would level Melendor so you have a debuff healer, and a non-purple healer for purple bosses/ titans. Melendor is a glass canon that does lots of board damage, so he does good if you ever get Wu Kong ( Note 2 ). His high healing ( 42% ) and high attack stat will also help you complete Rare Ascension Item Quests.
Merlin one of the best 4* heroes versus enemy healers. Also good against titans. But he is purple, so wait a bit on him. On a personal note, I started with Rigard, Sabina, Tiburtus and I never regretted my wife’s advise to level Rigard first.
Joon is insane with 40% accuracy debuff. But TWO 4* yellow heroes take 94% of the XP and the same amount of 3* ascension items as Joon. So leveling two decent 4* yellow would be my priority.
