The rebels/separatists OTOH had been boasting about owning a BUK system less than a week ago. It was also photographed twice by the Guardian reporters IIRC.
Any way:
-rebel/separatist social media behavior after the incident(we told you to not enter our airspace or whatever it was)
-crash site: rebel held territory
-Dutch report: particles/fragments that caused the destruction of the aircraft approached from ahead and from above the floor level: ie. from the East(rebel area and Russia)
-Su-25 not having a radar, not being able to carry radar missiles, at least some models not being able to reach that altitude at all, not being able to achieve that speed, carrying only short range contact-exploding heat-seeking missiles with a small warhead, and apparently the ones Ukraine has arent even all-aspect guiding and cant be shot head-on(shoots down the Russian theories propaganda)
-Dutch report again: immediate mid-air disintegration/explosion of the aircraft and no mayday call
-There has been an argument that the missile fired by a BUK system would have left a very long and visible smoke trail. This is IMHO only partially valid: the missile accelerates only 15 s(or less than the first 4000 m of the flight) and with its very long effective range even to such altitudes, the firing platform could have been very far away
I believe that these points more or less rule out direct Ukrainian involvement and make it very probable that MH17 was indeed shot down by a BUK or similar missile air defense system. Theres still the chance of an air-to-air missile, but Russia has not claimed Ukrainian fighters in the area(other than the Su-25 which actually isnt a fighter) and would have required the attackers to fly in an airspace where multiple aircraft had been lost at high altitude to surface-air missiles during the previous week. Also I'm not sure if R-27 has enough explosive content to do such damage. BUK's 9M38 missile has 20 times the explosive content.
Then theres of course the various radio discussions and such provided by Ukrainian military intelligence but I'm not sure how valid evidence they are or if any of them are even real. Russians and Ukrainians are both pretty good in "media warfare".