Destruction is one among the three activities that is undertaken by the Three Holy Manifestations of sanAtana dharma respectively. That being the case how could one say Lord Shiva who gets associated with the destruction be the Supreme God ?
braHma, viShNu, rudra, often referred to collectively as trimUrti, play the role of masters of creation, protection and destruction respectively. If we take this world as an example it is an easy task for almost everybody to involve in the acts of creation. (This is not to say that the Universal creation is a trivial task. All due respects to Lord braHma). Whereas as for as protection is concerned it is only few good hearted people, who have the strength and inclination to help for the cause of goodness do the protection deeds. But... not everybody is provided with the power of destruction. When anybody starts doing the destruction then there would be chaos which they call the law and order problem. There is normally one authority called government is given the authority to destroy and not others. This would show how difficult is the action of destruction and how carefully it has to be dealt with. This is the reason the act of destruction (to be correct this word should be actually "reduction", as things are not getting destroyed but only a change of state happens odukkam in thamiz and pralayam in saMskR^itam) is handled by a form of Lord shiva Itself, Who is called rudra. Since rudra is a form of the Lord, rudra is considered as Lord shiva Itself.
Ok, so far only one part of the question is answered. Agreed, the importance of the act of reduction (destruction). But when this Lord being one among the Trinity, how can this Lord be considered the Supreme ?
Let us take an example of a small shop. The owner employs a few skilled people to do the regular work and at peak hours or at critical periods, he/she also takes some important work and executes it. Now that individual wears two caps. One as the owner of the shop and other as the worker in the shop, but the individual is the same irrespective of whether he /she is acting as the owner or as a worker. Nobody questions his/her ownership because he/she shares the work load with the others in the shop. The same way though the Supreme Lord shiva is beyond the five deeds, It also takes roles in the critical deeds assuming a form specific to that role. So though Lord shiva gets associated with one of the three in the Trinity, He is the Supreme Itself.
A point to note here is that it is not just these three actions, but there are totally five deeds which are the acts of/on behalf of the Supreme. These five activities referred to as panycha kR^ityam are creation, sustenance, reduction, illusioning and blessing. The holy masters for these five activities are braHma, viShNu, rudra, maheshwara, sadAshiva respectively. Of these the later three are nothing but the forms of the Supreme shiva (called parashivA). So to conclude Lord Shiva is the Supreme, Who assumes various critical roles and assumes appropriate names and forms, and also stands transcending all these