Manaburn's really character dependant and can be massive fail against mauler builds. 
*Owls are pretty decent too on this regard but take way more micro, L6 owls burn 1K mana and provide truesight, along with this they offer no gold for dying so sending them into someone's base just to scout is fine.
Soulburn's more annoying than actually good IMO it also decreases movement speed and does damage, but overall is not worth a slot that could be silence.
Silence IMO is the best skill in the game at L5 and 6. L1-4 are decent skills as well. Late game makes you really hard to kill. Note, L6 silence can permanently keep a large group of darks from attacking... This is probably the best counter to darks in the game other than darks of your own (And of course transmute)
Hex is a slept on skill agreed, Hex also reduces the target's armor to 0 while they're an animal so units, golems, chims, etc beast even harder against them.
*Sleep is incredibly powerful and anyone who thinks 'just attack yourself with units' doesn't know about proper sleep use and how low its cooldown is, at L4 you can keep a full team locked down while farming their base. It's a double edged sword thanks to the time they're invulnerable for however.
Entangle is okay, again good support for things like golems, but also importantly is the absolute anti-mauler skill it stops attacking and their casting is limited if functional at all, people forget how useful this is against maulers --;.
Shadow strike I haven't used enough to form a good opinion of it.
As for other slept on skills that people seem to forget are good.
Quillbeasts: Stupid high DPS great as long as there are other things to tank for them.
D-shield: People don't give this enough love, but if you're a hero with a good/decent movespeed farmers can use this instead of windwalk in order to seriously deal damage.
Vengeance: This is practically as good as chims since the only counter to it the feedback counters just fine, if you have auras you can remove towers with it very quickly and fairly safely.
Tranquility: This is the only channeling ultimate in the game that doesn't suck. It's incredibly strong if your team mates have the right skills to take advantage of it.
Druids: They're exceptional HK and pretty good at everything in general, just need longer duration to be worth it.
Blink: With stomp farmers blink is really good as long as you have mana and health, I think d-shield is better, but blink allows you to get around quickly and its slow on exit is a good start even though at L6 it should be stun instead and 3 up should be slow.
the power of the irony