I have seen many scriptures to portray Lord Vishnu, Lord Shiva, Lalita Devi and Lord Surya to be the supreme. However, I would like to know whether any scripture portrays Lord Ganesha to be the supreme. According to Adi Shankaracharya, Ganesha is the one of the 5 deities that can give liberation. Also there is a sect named Ganapatya which sees Lord Ganesh to be the supreme God. What is the basis for their claim? Apologies to anyone offended by my question including Lord Ganesha.
Many scriptures including Puranas portray Lord Ganesha as Supreme Brahman.
Here in Ganesh Purana - Chapter 10 - Vyasa's Questions - Lord Brahma tells Shee Ved Vyasa that Shree Ganesha is supreme Brahman.
There is a person who has neither beginning nor end, who is the source of the creator the universe, who contains the universe, who is the controller and sustainer of the universe, including those who exist and the non-existent, the manifest and the eternal. He can, at any time, create, destroy or change anything. The demigods, led by Indra, are always subject to his command, and have great faith in him. He removes obstacles for his devotees like Vishnu, Rudra, Surya, Agni, Varuna and the rest, but creates obstacles for others.
If you do not think of Ganesha before any pursuit, be it a spiritual or worldly concern, or even when entering and leaving your home, he creates an obstacle. Those who explain the meaning of the Sastras and the Vedas speak of him as supreme bliss, as the supreme path, and as the supreme Brahman. Child! Go, and respectfully take shelter of Dviradanana. If the illustrious Lord is kindly disposed, he will do whatever you wish. If not, you will not obtain your desire in a thousand years”.
Gaṇapatistavaḥ which is said to be from Ganesh Purana - upāsanā khaṇḍa adhyāya 13 also says following. The stotra to Gaṇapati is by Brahmā, Viṣṇu, Maheśvara as reported by brahmā.
अजं निर्विकल्पं निराकारमॆकं निरानन्दमानंदमद्वैतपूर्णम् ।
परं निर्गुणं निर्विशेषं निरीहं परब्रह्मरूपं गणेशं भजेम् ॥1॥
Ajam nirvikalpam niraakaaramekam
Niraanandam aanandam advaita poornam
Param nirgunam nirvishesham nireeham
Para brahma roopam Ganesham bhajema
Brahmā Viṣṇu and Maheśa spoke: We should worship Gaṇeśa, Who is unborn, Who is free from change, Who is beyond taking forms (or actions), Who is unique, Who is without a support, Who is without a second, Who is bliss, Who is complete, Who is supreme, Who is beyond qualities, Who is absolute, Who is without passion, Who is supreme, and Who is Brahman. -
You can read full Stava - Here
Also our famous Ganapati Atharvashirsha (Ganapati Upanishad) says so.
त्वमेव प्रत्यक्षं तत्त्वमसि ।
त्वमेव केवलं कर्ताऽसि ।
त्वमेव केवलं धर्ताऽसि ।
त्वमेव केवलं हर्ताऽसि ।
त्वमेव सर्वं खल्विदं ब्रह्मासि ।
त्वं साक्षादात्माऽसि नित्यम् ॥2॥
Tvam-Eva Pratyakssam Tattvam-Asi |
Tvam-Eva Kevalam Kartaa-[A]si |
Tvam-Eva Kevalam Dhartaa-[A]si |
Tvam-Eva Kevalam Hartaa-[A]si |
Tvam-Eva Sarvam Khalv[u]-Idam Brahma-Asi |
Tvam Saakssaad-Aatmaa-[A]si Nityam ||2||
(O Ganapati) You indeed are the visible Tattvam (Conscious Essence underlying everything), (O Ganapati) You indeed are the only Creator (Karta) (by Whose Power the Universe is Created), (O Ganapati) You indeed are the only Sustainer (Dharta) (by Whose Power the Universe is Sustained), (O Ganapati) You indeed are the only Destroyer (Harta) (by Whose Power the Universe is finally Dissolved in its Conscious Essence), (O Ganapati) You indeed are All This (The Universe); You verily are the Brahman (giving Consciousness to All), (O Ganapati) You are the visible Atman, the Eternal (underlying Reality).