The fact these creatures farm Humans is perhaps and indication of their superiority. The only way we manage to survive encounters is by using stolen and gifted alien technology, diplomacy is one strength we possess, although until the world can live without war then we may not be ready to stand united to show this.
The Vulcan species are very intelligent, they are methodical and have become enlightened due to their astute intelligence. The lack of emotions, is often their downfall but being peaceful we don't have to fear an encounter with this style of race. What we may find, is that brute force is not the only way to conquer a species, if they were to integrate it can be assumed they would assume powerful positions, due to their superior intellect.
This species has moved from sworn enemy to ally, showing that diplomacy can work over time. One of the key aspects of diplomacy is to ensure both sides are strong in their own ways, if present day humans met the Klingons we could not expect such a positive outcome.
The asgard offer an olive branch on first encounter, ensuring our transition into interstellar adventures is protected by their strong technology. Often a peacekeeping voice of reason, they are thought to be the kind of race that humans may evolve into.
Sebaceans are very like humans, being from a different universe they are like a slightly altered version of our race. The superior race of their universe they have assumed a role of peacekeeping, unfortunately due to the corrupt power base they have taken a rather dictatorial approach to this peace keeping.
The Scarran species are evil, powerful and arrogant. Lizard like in appearance they possess strong natural powers, lacking in intelligence they are perhaps another race we are not ready to encounter.
The Borg are a cross between the terminator style future digital race and hugely intelligent AI. This would perhaps be the most likely type of encounter, with a race that is spawned from our own fumbling in scientific research.
The Daleks have been a long time foe of the Human race, brought to us by another species the Timelord. The side effect of first contact, they may not be alone and any grudges they bear could cause untold harm on our own species.
Not technically another species, but an example of what happens with regression evolution. Humans revert to animal like behaviour, as we know Humans are dangerous, more so when you remove emotions like guilt and they begin to act purely on compulsion.
Man makes robots, robots make man. This cycles of development is easy to understand in terms of evolution, if AI is advanced enough with such human reasoning to create life, it stands to reason this type of species will one day exist. If your in charge of global security, ask yourself why that Hot girl is sleeping with you and call her a taxi before you go to sleep!
In a galaxy far far away this band of evil dark lords are powerful and dark. Great at hand to hand combat, but rely on a corrupt leadership to bankroll their exploits.
A highly inefficient race, that is obsessed with procedure and paperwork. The ultimate organisors of the universe, but don't ever invite them to open mic night!