For low-rarity units, the best ones for farming are Arash, Spartacus, Paul Bunyan, and Kiyohime. Of course there are other units you could use, one strong choice is medusa for farming QP, but those three are the best because of the NP charge skill on arash and spartacus, and the fact that spartacus, Paul Bunyan, and Kiyohime are berserkers so you can use them against any enemies.
For other things such as boss killing, dps, and support, the area is a bit more broad.
For Single Target riders/quick dps, Ushiwakamaru is great, especially considering her rarity. She can even keep up with most 4 and 5* units.
Georgios is a great taunter/tank and fits especially well into arts teams, the only downside he has is that he absorbs lots of critical stars due to his rider class.
Robin Hood is a great Single Target archer with an arts focus. His nuking potential is high due to the additional special damage on his NP and he fits nicely into arts teams.
Hans is a good all-around support even though he is a bit fragile due to being a 2*
Cu Chulainn is a great tank and can clutch any kind of fight for you if you need him (as long as the boss isn’t a saber)
Although you already have Penth leveled, Lu Bu is a fantastic ST berserker with really high burst damage.
Euryale is another good ST archer who specializes in anti-male antics.
Hassan of the Cursed Arm is a really good star generator and has fantastic survivability in case you’re planning on running any kind of buster-crit or quick team. Fuuma is also good but he dies quite often due to not being as durable as Cursed Arm.
Hassan of the Hundred Personas fits into arts teams as a nice damage dealer. She can deal good damage if she gets her buffs off and has decent NP gain as long as her Librarian of Knowledge skill is up