Bhūloka Vaikuṇṭham
Śrīraṅgam (Tamil: ஶ்ரீரங்கம் — 'the divine stage/amphitheater of Śrī') is universally venerated in the Śrī Vaiṣṇava tradition as bhūloka Vaikuṇṭham — 'Vaikuṇṭham on earth.' It is the first and foremost of the 108 Divya Desams, called simply Kōilam (The Temple) by devotees — when Śrī Vaiṣṇavas say 'Kōilam' without qualification, they mean Śrīraṅgam.
The Lord: Raṅganātha
The presiding deity is Śrī Raṅganātha — the Lord Śrīman Nārāyaṇa in His divine reclining (śayana) posture on Ādiśeṣa, the cosmic serpent. Raṅganātha is the śayana form (as opposed to the sthānaka/standing or āsana/seated forms at other Divya Desams). The Tāyār (Pirāṭṭi) is Śrī Raṅganāyakī.
The Temple
Śrīraṅgam is a temple-city (kṣetra) spanning 7 concentric enclosures (prākāras) covering 156 acres — the world's largest functioning Hindu temple complex. The outermost enclosures contain residences, markets, and institutions; the inner enclosures house the processional paths, mandapams, and ultimately the garbhagṛham (sanctum) of the Lord.
Historical Significance
Śrīraṅgam was the center of Rāmānujācārya's life and mission — he reorganized worship here and established the tirunāḷ (festival) system still in use. Piḷḷai Lokācārya saved the processional deity during the 14th-century invasion. Maṇavāḷa Māmunigaḷ conducted his famous kalakṣepam here. All the greatest Āzhvārs and ācāryans have connections to Śrīraṅgam.