Most viable F2P unit

For arena, I'd say Fallen Takumi (with C Duel Infantry). Otherwise, it's whoever you like most. They're all good units.

Aversa and H!Dorcas will be exceptional once they're available so you might want to save for them if you're interested.

i'd say masked marth, ethlyn, garon, m!kana, and a few others like maybe arvis and camus are pretty strong, if you dont like armour units.

It's hard to say, a lot of units were in the camp of 'great but free'
Putting arena score aside.

The units that want merges the most, are the ones that make use out of multiple stats well. Allround units.

Camus is a good example, he has use for all his stats. Most of em fall just a tad short compaired to other lance cavs, but with some merges he quickly reaches a level that allows him to reliably deny doubles, deal good damage, tank multiple hits and take damage from mages.
Him having Gradivus gives him a niche that's much more easier to make use out of.

Joshua is another unit. Every stat above 30 (with weapon) he's a great unit for ploys, dragon dueling, physical dueling, DC even.
His speed was at first just not good enough, but with merges he can accually do some good.

Walhart might be the best. If there's any unit that's stat dependend, it's him. Amazing atk and bulk with his weapon, but speed that's just short of useful. With refines he can reach levels of armored units with 3 movement. Just keep him solo
Speaking of that, he'll make great use out of any solo skill. Speed is greater for him than it seems. And Fortress def/res is also a great option.

Lastly I'd say Arvis.
A great allround mage, and an amazing support.

But!
You might wanna wait a bit till 3.0 (december) before doing anything drastic. It's very much possible some of these units will get refines. Which changes who'll be the best picks.

Units like Lyon, Berkut, Arvis, Saias, Narcian and Robin are all very likely to get a refine in the future
And 3.0 is the biggest update where there's a chance.