Since no skilled impartial forensic analysis is likely, or even possible, I will go through the circumstantial evidence impartially with no pre-conceived ideas, to see what insight I get. The things I will use as evidence are signature of the crime, possible motives from entities that match the signature, credible claims of responsibility or denial, opportunity, and a quick game theory check that the results make sense.
Signature of the crime: Analysts are in no doubt that the signature of the crime strongly suggest a secret service organisation "hit" and almost certainly of a state backing/state ordering variety. This rules out a random killing, opportunistic attack, or even a terrorist organisation therefore.
Claims of responsibility: A previously unheard of islamic organisation is said to have claimed it, but it seems likely to be a red herring. Syrian government officials have denied all knowledge or involvement in the assassination, and seemed genuinely surprised at it happening.
Motive: At first sight, the Syrian government would have a motive, as Hariri was openly campaigning for Syrian withdrawal. However, the consequences of the killing were predictably against the interests of the Syrian government, so you would have to assume irrational decision making. If you had such a complex and long running assassination plot, you would have to be sure the results would go your way. Non-intuitively, therefore, the motive would point to a non-arab country. As to which non-arab country, I will move to opportunity.
Opportunity: The smidgen of reliable information of the crime scene points to a pre planted explosive within the road itself, set off via remote control. Therefore, the culprit organisation would have been posing as a roadworks/utilities contractor. There would likely be several planted explosives at various locations waiting for the right opportunity. As Syria has been controlling the country rigidly for the last several years, American or Israeli companies would have drawn too much suspicion and would not have had operational access. That pretty much leaves Russia and France as the most likely perpetrators. I'm leaning towards France due to their historical association with the country