Summoner's War - Sky Arena

I think splitting ranking is a mistake in general, because with one notable exception (Water Imp), just about anything that's good for Dungeons is good for Arena Offense/Defense as well.

SUPER ROUGH LIST OF THINGS WORTH KEEPING (Unknown Scroll Edition):

Fire:
Yeti - Good PvE tank, can do a decent Ramagos impression against certain single Wind mobs.
Hellhound- One of the best buffers in the game. +Atk/Crit is a huge offensive boon and a big part of Giants farming for many players.
Harpu - Good all-around utility unit, with an anti-healing debuff and a heal+attack boost.
Fairy- Great AoE for a low* unit, with CC potential on turns where her main AoE is down.
Inugami- The game's premier single target shitwrecker. Very few units have the one-turn damage potential of Raoq, even if it's very RNG-dependent.
Werewolf - Tanky, I guess? He's probably the weakest fire unit listed here, though he's very difficult to kill with healing backup in a Wind/Water dungeon.
Viking - Offensive dispeller. Not fantastic, but there aren't many units that can do what he does.
Grim Reaper - Provides an attack leader bonus while awakened, and Living Hell is a strong AoE with a lengthy anti-healing debuff.
Living Armor - A tank that gets to double-dip Defense -> HP at a 4:1 ratio. Both of his abilities scale off of Defense and are quite useful. A strong contender for best 3* Fire tank.

Water:
Imp- Basically like this. Required for most schemes that clear Giants B7-10.
Howl- Consummate pure healer. Absolutely no offensive pressure, but two great %-based heals and increased speed when he's not healing.
Garuda - No personal experience, but Resurge is a powerful tool & it has a full team dispel, so it's good enough to look at.
Fairy- Great arena unit, offensively & defensively. One of the few units with a power-based heal worth a damn.
Amazon - Good AoE that can steal out a decent number of wins.
Inferno - Adrenaline is a very good skill for Giants 7-10 if you have access to it, but Infernos are slow.
Epikon Priest - The single most annoying defensive unit in the game. Not the best, but the most annoying. Does absolutely nothing.
Werewolf - decent support unit, sorta meh like fire WW
Mystic Witch- Excellent PvP / PvE support unit, but sort of weak on her own. The best 3* and lower buffer, by a decent margin.
Bounty Hunter - High burst potential + speed boost. Above average Water single target attacker, with Raoq-style burst potential.
Grim Reaper - Decent Water attacker, with a guaranteed stun and a +Attack leader skill. Very slow, though.
Living Armor - Above average defensive unit, with a passive that makes him situationally incredible for certain monsters and Secret Dungeons. Something like Water Imp has a really hard time actually taking a Def-runed Nickel out.

Wind:
Imp- Ralph does entirely too much damage, and will 1-shot equivalent units for a loooooooooong time. That's all he's good for, though, because he makes Hellhounds look like Rina as far as defensive stats go.
Pixie - Probably the second best "free" support unit. Pep Talk and Decelerate combine to give her great supporting potential, but not much to speak for on offense. Her greatest weakness is being the same element as Ramagos.
Yeti - Solid healer for Secret Dungeons, with good support skills. Has the same problem as Pixie in that Ramagos can't effectively tank for him, but he can be very strong in PvE and Arena in stall comps.
Warbear- Get one. Even if you pull a natural 5* unit, there's a good chance that Ramagos will be your best monster until you've awakened it. Ramagos is unreal powerful and can solo a huge chunk of the game's content with good runes, and he's one of the best Arena offense monsters in the entire game to boot. Clean Shot can easily clear 15k damage even at 5* level, and Crouch as a followup makes it a reliable nuke.
Griffon - Great +Speed support unit, with decent offensive capabilities. I don't have much experience with this unit, except that the Wind Griffon teams in Hard/Hell Tamor held me up longer than almost anywhere else in the entire game.
Epikon Priest - Eh. Worst Epikon priest. On the right team, Soul Revival can be obnoxious, but I don't really know why you'd want to use her over other options. Don't trash her, though, because you need her for Chloe skillups.
Magical Archer - ??? No personal experience, i've heard she's quite strong though. Good +Attack leader skill.
Living Armor- Can apparently solo Dragon Keeps once you get him super kitted out. No idea how that works, but that seems pretty good.

Bold means extra good.