No definitive proof for me yet, but a lot of it is pretty sound. I believe we are not alone... it's just a matter of proper First Contact being made which will probably never happen because humans, especially ones with power in the world, are violent and destructive. They fear what they do not understand... which is also a basis for much hate amongst ourselves all over the planet, sadly. Me, I'm non-violent, non-competitive, easy-going, and open-minded. If an alien ship wanted to touchdown in my yard right now and say 'hey' face-to-face without all the abduction jive, I wouldn't be opposed to it.
Crop circles can be easily forged, in fact, I don't even consider those when looking for alien evidence.
Abductions? A few, I'm sure, could be substantiated but not all of them. From what can be gathered from many of the more documented ones, there are some that are hostile abductions, and some that are peaceful; one documented that was so harmless that the aliens supposedly disembarked and led the abducted couple on-board in a non-threatening manner. This always makes me think of the theory that there is more than one faction of alien visitors. Ones that want to exploit us, and ones that try to protect us. Again, just theories.
There's a high chance that we've had visitors here in the past who have directly intervened in our world's civilizations. Several examples can be found with the Egyptians and Mayans in their ancient writings and carvings. Where we dismiss this imagery as Pagan Gods of less sophisticated human cultures, we never stop to consider that aliens descending from the sky came across as celestial beings to these people and were, hence, held in that regard.
And UFO sightings seem to be more and more frequent in recent years. There are skeptics who dismiss them sometimes as supposed "test flights" of new military crafts. These same skeptics, who desire proof that it was an alien UFO, will take what the government tells us as the gospel truth. To that I say that I want to see proof that it was a military test flight. Not saying that I'm convinced it was alien, just that I want proof for ALL sides of the argument. Of all the UFO witnesses out there, the most trustworthy sources would have to be airline pilots. They know what is and isn't normal in the skies, if they described something uncommon to this planet, you can rest assured that it's 90% true.
I don't know though... I guess I'm just a naive futurist that only wishes to see the world get over itself, grow up, and join the galactic community someday if there even is one.