Amber 7* is a D speed missile with high air attack. She'd be a good specialist for events like Dice or Legion, but her speed would be a liability for a number of other events. Her low deployment cost is useful for events where you have a team of units, but those same events usually feature speed as well.
Lulabelle 7* is a B speed melee with high ground attack. She's also a good specialist, but her speed helps her be useful in other events as well.
Ruby 7* is the same D speed missle that Amber is, but with even higher air attack, and Area Attack as well. She's a good specialist for events that don't feature speed, but her range and skills make her useful for tower defense (read: quests) type events as well.
I think you'd get the most use out of Lulabelle, but Ruby looks like a good unit too. Amber starts to look more useful in events where having lower deploy cost is an advantage (same for Bernadette and Ninette).