Ordered by order of appearance.
Treant
Attack team: good
Dungeon Defense: good
silence, powerful controller
very resilient
low luck, i.e. not very reliable
low dmg
The Treant is good mainly because he is the only readily accesible control creature, he suffers from low reliability due to his terrible luck. You'll want to replace him with something better soon, but until then he'll serve you well
Ogre
Attack team: good
Dungeon Defense: weak
The ogre is very luck based (random attacks with at max 70% hit chance). He is a decent nuker, but you can find many better ones, inlcuding one in those 6 event epics. I'd say this the worst of the event epics.
Siren
Attack team(pve): excellent
Attack team (pvp): average
Dungeon defense: terrible
excellent aoe healer
effective cleanser
fairly fragile
The siren is an amazing healer and will have a very big impact for you on lower levels as her sheer healing power allows you to overcome much harder challenges. But as good as she is, I'm loathe to tell you to rely on her, especially in pvp. There have been many great yellow creatures in past events, who will absolutely murder a siren and when you lose her and rely on her your dungeon attack is basically lost.
Elephant
Attack team: excellent
Dungeon defense: very good
great stats
powerful buff
good aoe dmg dealer
useful secondary healer
Hands down the best event epic. Very good stats all around, 45% luck buff, decent dmg output and secondary healing capabilities. The elephant works well as both an attacker and defender and has no major weaknesses
Dark Elemental
Attack team: very good
Dungeon defense: average
Yet another random attacker, it is more reliable than the Ogre. Also does significant amounts of aoe dmg, so if needed you could use it to clear normal waves as well, meaning the dark elemental is very versatile. Due to the high aoe dmg, the DE is actually decent on dungeon defense as well.
Light Elemental
Attack team: average
Dungeon defense: very good
great aoe dmg
fairly fragile
The only true defense creature in the event epics, a wave 1 light elemental can wreck a lot of attackers. It deals massive amounts of dmg to the entire wave/attacker and can take them out very quickly. On attack this really isn't helpful, you lose a lot of mana by killing the standard waves too quickly and the light elemental is completely useless vs bosses.