Samuel - Rare (high level): Endless self-healing, less need for a babysitter and allows you to leave out the armor for more damage.
Grace - Legendary: self explanatory. The heal reduction isn’t that much, especially when you throw in a crystal item or two.
Adagio - Rare (high level): ENDLESS HEALING SPAM.
Flicker - Epic (if you’re not building captain): The speed boost is very disengage-friendly, and can help you zip around to help your teammates.
SAW - Rare (high level)/Epic (generally): Rare is great for bunched up enemies. Use high level for when you’re up against idiots with no concept of spacing. Generally Epic is better/best because it completely negates his slow. It turns SAW into a complete beast with no downside.
Idris - Rare: Best of both worlds. Build CP for untimate cancer.
Lyra - Epic: Use against Joule/Taka for instant “top rage quit compilation”.
Ardan - Epic (high level): How to one-shot everyone 101. Build WP for ultimate cancer.
Catherine - Epic: Stun train OP.
Gwen - Rare (CP, high level)/Epic(WP): Rare is better for max ability spam, Epic is good for more skillful use of abilities. Allows you to focus on damage without worrying how you’re going to get away/kite.
Joule - Epic: Use with Echo, SG, and CW to ult every 2 seconds and melt everyone.
Phinn - Legendary (high level)/Epic: Use legendary if you have two hyper carries. Soak up the damage and let them both backline from behind you. A crystal-heavy Lyra and a carry will also work. Her emergency healing and dive-deny is a big help. Epic is all-around great for diving and disengage.
Krul - Epic: Use in case of “both my teammates chose squishies and now I’m a babysitter”.
I have no opinion on the rest, mostly because I don’t play them.