I'm going to ignore "don't bother with reds", because Olivia is there, and you 100% want to train a dancer. Xander is pretty good too, though you need to promote him to 5* for him to really work, since that's when he gets access to Siegfried and its built-in Distant Counter. The Falchion foursome are good because of Falchion's built-in Renewal; give them Reciprocal Aid and they can act as combat medics. Lucina is probably the strongest of the three, overall, though Alm and Marth have nice bulk, too. Chrom probably makes a better Brave Sword user, though, given that he has the highest Atk stat of all sword units. And you'll make
Opossum
happy if you use him. What's not to like?
Moving on to blues: Effie has huge Atk and solid Def, but is slow so she's probably best served by getting a Brave Lance+. Likewise for Est, but she's far more expensive to build. Ursula can be good if you give her Blárblade from Odin, and pile buffs onto her.
will be very useful to you once CYL Lyn shows up everywhere, since his Blárraven tome gives him weapon triangle advantage over colourless units; give him Triangle Adept for best results (which you can get from a regular 4* Roy, or Selena). I like Catria and
a lot too; Catria is a solid melee duelist with a good default skill set that just needs a few tweaks to shine, while Corrin is great for debuffing and tanking, which I think are useful in some of the higher level content they've been adding over the last few months. Subaki is good too, but everyone just uses him for Quick Riposte fodder, since that skill is really useful on slow, bulky units like Xander. And I suppose I should mention Cordelia, who comes with a Brave Lance and the stats to use it, too. I don't like her art, though.
Greens: Cecilia is good for the same reasons as Robin; her Gronnraven+ tome gives her advantage over colourless units. Again, she ideally wants Triangle Adept to work best. She also comes with the added bonus of being effective against Reinhardt, who has usurped Takumi as the game's most feared enemy. You can also just stick Gronnblade on her since blade tomes are dumb. Nino also comes with Gronnblade, and makes really good use of it. As with all blade tomes, they both need to have someone alongside them piling on the Hones and Rallies, to do their best. Soren can pull off Gronnblade too, but is much more expensive to build than Nino, so it probably comes down to which character you like better, and which you feel is most deserving of the investment. Merric gets a lot of flak, but I like him since bulky mages are satisfying to use, for me. Camilla is probably best used as Brave Axe+ fodder if you ever find a +Atk Cherche. Or if you really want to use her, I'd probably go with an Emerald Axe+ since her Atk is just too low to pull off the Brave Axe.
Colourless: I think everyone ought to have at least one Brave Bow archer in their kit, and both Gordin and Klein can pull this off with little investment. Gordin has the added bonus of being nice and bulky for an archer, but Klein comes with Death Blow as standard, which has great synergy with brave weapons. Kagero's weapon gives her effectiveness against infantry units, and infantry units are many, so she's worth using for content outside the Arena (in the Arena, non-infantry teams have mostly taken over, so she's less useful there). Felicia has one of the highest Res stats in the game, which makes her great for tanking magic hits, but her Atk is pitiful so she's reliant on her special to deal damage; luckily, she comes with Glacies, which is really powerful on her (shame about its slow cooldown). Healers in general are lame, unfortunately, though it's worth training up maybe one or two for modes such as the Chain Challenges, where sustainability is important.
Some of these units can turn out sub-optimal depending on their boon or bane; Lucina will struggle if she is -Spd, and any brave weapon user will ideally want to be +Atk, for example. Though ultimately I believe that you can make any unit work regardless of boon or bane, if you really want to.
You should probably train Alfonse, Sharena, and Anna, just to ensure you always have a bonus unit available for the Arena. Those three rotate on and off every two weeks, and at least one is always one of the bonus unit options.
There are probably one or two strong options in there I didn't mention that someone else might spot, but I'm fairly sure I covered most of your best bets.
Overall it's probably in your best interests to just train as many units as you can. Most of the content they've added in the last few weeks favours having several strong units available, instead of just one overpowering team. So saying you won't bother with reds just because you're getting CYL Roy won't really work out, given how the game has changed since you left.
Anyway, that went on a bit longer than I expected. I hope there's something useful in there for you.
