Who is called Purshottam in the Vedas?

 In Gita Bhagavan Krishna says

यस्मात्क्षरमतीतोऽहमक्षरादपि चोत्तम: |
अतोऽस्मि लोके वेदे च प्रथित: पुरुषोत्तम: || (Bhagavad Gita 15.18)

I am transcendental to the perishable world of matter, and even to the imperishable soul; hence I am celebrated, both in the Vedas and the Smṛitis, as Purushottama.

Now we have reference that call him as the Supreme Purusha(Purushottama/Parama Purusha)

ॐ यो ह वै श्रीरामचन्द्रः स भगवान् यः परमपुरुषो
भूर्भुवः सुवस्तस्मै वै नमो नमः ॥ (Rama Tapini Upanishad 2.4.41)

Salutations unto Sri Ramachandra who is the Supreme Purusha.

ॐ योऽसावुत्तमपुरुषो गोपाल ॐ तत्सद्भूर्भुवः सुवस्तमै वै नमोनमः ॥ (Gopala Tapini Upanishad 2.3.14)

Lord Gopāla is the Supreme Person. Obeisances to Him.

Hence, Hari is the Purushottama of vedas.