I think the life minion is. I may not be life but i have a friend who summoned one and it kept healing me and him. So far it is the only unselfish minion i have ever seen! I really like the fact that it heals you so you don't have to fill your deck with heals, you can just summon it and it will heal for you.
I'm a myth wizard, so I had access to minions from the beginning. I think that although I haven't got a chance to see other school minions, I think the cyclops minion I got from a level 18 quest is quite helpful. He has a number of different spells other than just myth including sprite, storm shark, fire bird, power play, and one of those death spells that drains enemy health and gives half to the caster. He also has blades and traps for myth, so I think that he is quite good. But since I'm going to get a minotaur minion later on, he probably isn't the best.
The Guardian Sprite is awesome! She'll usually keep you healed up in just about any tough battle. The downside is that she's not very bright -- heals and charms are given pretty randomly. Also, she has a tendency to set off your traps.
The other one I like is the Storm Minion for its ability to steal aggro from the entire enemy line and take the damage as well.
Personally, I don't find Helpful Mander all that helpful.
life sprite has low hit points i can kill it in one hit basically it is helpful in lower tier levels basically..
minotaur is the best for any i mean any boss battle.. because he uses taunt all the time and has absorb shield as well as tower shields one time i summoned a minotaur and he had over 9 tower shields and 3 absorbs.. and whats good is that minion never runs out of cards..
as for pvp i am not sure death minion is good because of the 900 hit points takes maybe 1-2 hits depending on what you use.. as for me earthquake and spirit blade and myth blade and field card does the work and i kill any minion with earthquake
:) :D 
