[Enchilada]

IMO, Abu ʿAli al-Ḥasan ibn al-Ḥasan ibn al-Haytham, westernised as Alhazen (which I've known him as) - the same name as the crater named after him on the Moon. Yet researches haven't published his remaining astronomical works as English translations as yet, and until they do, it is hard to place his position in the scheme of things. He might have been brilliant, but his influence on the evolution and rise of science as we know it today is - well still is somewhat questionable.
It is also his Arabic predecessor Al-Sufi (903-986AD) was probably more influential on the development of astronomy. If it weren't for him, Ptolemy's Megale Syntaxis would not have survived in its entirety. Al-Sufi also updated Ptolemy's work, that included stylised drawings of the constellations. He also observed several then unknown new deep-sky objects.
As to the earliest or scientist is surely debatable, mainly because they were really natural philosophers, whose interest was in the natural sciences and the uses practical of the applied sciences. Examples of here of such applications would be people like the physician Hippocrates of Cos II (460BC-c.370BC.) and problem solving of alleviating sickness or illness. Understanding of the practical sciences was high on the agenda in Roman Times, and there are many examples, but likely the best we know of is the engineer Marcus Vitruvius Pollio (c.70 BC.-c.15 BC). His books - the De Architectura became the textbook on the use of materials and general building construction of, say, public facilities and temples, etc. He also had good understanding of astronomy, whose translated writings are very well written and useful for very basic surveying. Were both of these scientists - well yes and no. They used methods of deduction and a crude form of the scientific method, but some of their ideas were based on philosophy and religion.
If you ask me, I would say probably the astronomer and mathematician Hipparchus of Hipparch, who built an observatory on Rhodes in the Mediterranean, and did careful observations of the star positions never done before. However, as much of his writings has not survived, it is hard to adjudge his contributions. Others on a similar par would be say Thales of Miletus (his works on geometry) or Geminus of Rhodes.
Really, the first scientist was likely Isaac Newton, based solely on his volume of work independently discovered and employed. If not him, then it was the application of some theoretical science using the modern scientific method, which rose in the 1850s. This is when natural philosopher became what we term a "scientist" - often defined as "...a person who is studying or has expert knowledge of one or more of the natural or physical sciences."
IMO, all of the above mentioned meets this criteria.
Was Alhazen the first physicist? well pity this wasn't the original question.