SQUATCH! But seriously aliens will be discovered first.

Aliens, which will then dispel the myth of God (the conventional definition of it anyway).

Aliens most likely

Probably god in a totally non religious form.

Maybe aliens, but it may take a lot of time.

None of them.
Aliens could exist, but it's highly unlikely to discover them.
The other ones are fantasy or impossible to discover.

Aliens are the only thing possible or likely.

God is a creation of man. Man created God to explain the things he could not understand.
Yeti's don't exist. They are neither our imagination or a fiction, just a misunderstanding of what was at one time.
Ghost are a figment of our imagination. We created them to explain what we cannot process.

If aliens exist, and in some form they likely do, they would likely be no more superior of intellect or intelligence than we are. Our world alone is 4.5 million years old and we are this evolved and this intelligent. While the universe is twice more older (13 million years old) you also have to take into account the ability for those civilizations to survive resource depletion, local star aging, or a collision with an object in space, such as an asteroid or comet. And they would have needed to do so in an even younger universe filled with far more upheaval than we currently experience today.

So, if aliens do exist, they will likely not be far superior to us and more likely less.

I did a paper in school regarding Bigfoot.  I strongly believe the aboriginal sightings of him can be attributed to sightings of the North American Short-Faced Bear (Arctodus simus).

The bear was a monster, at 6' tall when on all fours, they could reach a height of 15' in a stretch.   They were larger than the Polar Bear.  An adult on all fours would have been formidable, but standing they would have been frightening as hell. 

The native American tales regarding sasquatch (big foot) were similar to how a bear would have acted.  Keeping at a distance from fires, just outside the range of vision.  Standing, being able to look down into tee-pees.  Scavanging for food in camps at night.  And no wonder the latter.  By the time the first humans came to North America, the food supply for the short-faced bear was diminishing.  Then, to add to the difficulties, humans were hunting for and making extinct their same food sources.

Within 2,000 years of man's arrival in North America, the short-faced bear was extinct without a single exsiting oral or cave painting record of man's interaction with one.  How do you have a fossil record of a bear the only human species has no record of interacting with, but there are tales from multiple tribes of encounters with sasquatch, for which there has never been a single fossil record found?

Aliens

Gravity forces me to believe that it exists.  Knowing that something exists wouldn't deptrive you of free will.

Also god exists. It's just a particle though.

Which god are you talking about specifically? Humans have created about 10,000 different versions of that idea over time.  

To answer the question, alien life. I think it's probably inevitable.