Hari OM:

There are many discussions on this topic and debates "who is Supreme"

Before we can debate who is supreme, i think first we should understand who they are?

The unifying concept of various confusion can be achieved, if people understand the concept of "Saguna Brahamam"- The Brahamam without name and from but with Charactersitics.

In Hinduism, unlike other religions, The Creator (God) and the Created (Universe) are not seperate entities, seperated by Space and Time.

Rather it states that the One Saguna Brahamam, manifests itself as the infintenly diverese universe.

The earliest manifest are Brahma, Vishnu and Shiva.

There is a concept like "Person-Principle" Duality where there is a Principle (Say Space, time) and a Person (whom we can call the controller of the Principle), the Person is generally differenated from the principle by adding "Maha" before the name.

Principle-Wealth-Name Laxmi
Person-Wealth Controller-Name Maha Laxmi

Principle-Space-Name-Vishnu
Person-Space Controller-Name Maha Vishnu

Principle-Ego (or mind)-Name Indra
Person-Name-MaheIndra

etc.,

The first three manifesations (all manifestations are dual, with a person-principle) are Consicouness-Brahma, Space-Vishnu and Time-Shiva

"Prayajana Brahama" - Consicuness is Brahama (one of the Mahavakiyas of Veda)

Vishnu-literally means one who is every where, and only Space is every where, any other object can be only some where

Time-Shiva- Generally in North India, Shiva is referred as Kal-Ka-Ji, the master of Time.

From the three various persons manifested (or born out of will), From Brahama-Indra, Prajapathis,... From Vishnu-Sanka and other sages (stated in Gita-Sanaka and their four olders are born out of My will), From Shiva- Yama, Subramanya, BudhaGanas,etc.,

As well each of the trinity also manifested as Avathars (in specific form for a specific period of time for a specific purpose)- there are 5 types of Avathars.

People are allowed to worship different Gods or Avathars of Gods they like, in fact in many of the Yagna or Poojas all their names are listed like- Ishta Devata (Devata of your choice), Kula Devata (Devata of your Kula), Parampariya Devata (Devata of your traidition), etc., as well as Avathars of the Gods.

And naturally many people wanted to directly worship the "Sarguna Brahamam" itself along with their Gods. Since atonment with Unmanifest is difficult for people in Body (as stated in Gita), and since "Sarguna Brahamam" is not going to object if anybody assigns a name or form to it, people assigned name and (mystic) form to IT and now they can easily Worship.

Due to various Gods, people assigned their Ishta Devata Form and Name (or very similar) to the Sarguna Brahamam and started worshipping IT.

So Shivites named Sarguna Brahamam as "Mahashevra" and assigned a form similar to Shiva and described Brahma, Shiva, Vishnu Manifested from Him and Hence He is the "Supreme God"

Vaishivites named Sarguna Brahamam as "Narayana" and assigned a form similar to Vishnu and declared him "Supreme"

Shakities name Sarguna Brahamam as "ParaShakthi" and assigned a from similar to Shakti (or Parvati) and declared her "Supreme"

and so on...

Since this became very huge, and people forget the concept of "Sarguna Brahamam" and became more interested in the name and form concept, Adi Shankracharya combined the major six Ishta Devatas (Shiva, Vishnu, Shakti, Ganesha, Subramanya and Surya) and Created "Shan matham" where he stated that the "Sarguna Brahamam" is in all the above six forms simulatenously and people can worship any one of the form.

To emphasise this point there are slokas like "Shivasiya Hradiyam Vishnu, Vishanesha Hradiyam Shiva, Yeda Shiva mayam Vishnu Yevam Vishnu maya Shiva" HariHara Concept, Shiva Dancing in Vishnus Heart (in Thiruvathoroor Tamil Nadu- i think this states the Space-Time unity), etc., etc., so it would be better to follow the established norms and understand the Brahamam concept instead of debating who is Superior

Please note all the above are my own speculation by reading different Hindu and science literatures, please don't ask "Proof" from me.