The ones I focused on first were Lucia for melee healing slot, Sabrina for ranged damage, and Helena because I love using Vanguard Tactician as my first unit once they are awakened. I also CC'd Bestla, but am not sure if I'll keep building her long term based on my other units.

Back in the Nutaku days I leveled and used Odette, Fran, and Kanon fairly regularly, but I haven't touched them other than combining teh farmed units from revival in DMM. As with the above reply, it really depends on what other units you get with your summons as to what can be replaced and what you keep. If you have any questions about specific units, just ask.

I'm still only 2 months in on DMM, so take that for what it's worth.