Kailash.

परम रम्य गिरिवर कैलासु | सदा जहाँ शिव उमा निवासु ll 

Param ramya girivar kailāsu l sadā janhā shiv umā nivāsu ll

Of all mountains the most charming and the best is kailasa, where Shiva and Uma eternally dwell.

Baal Kand, Doha 105, Sri Ramcharitmanas.

There are four main reasons Kailash for which Lord Shiv favours Kailash. 

• Uchai: Kailas is situated at a height, it cannot submit itself to or accept lower thoughts. Thus it is untouched, only Shiv Parvati dwell here.
• Sthir: Kailas is stable; Shankar is not fond of instability and unstableness. As with Kailas, Shiv will never loose his patience (dhairya) and stable stability of mind (sthairya) regardless of how much one misunderstands and views Him.
• Sheetal: Kailas is cool and pure; Shiv is cool by nature, never fiery. With his agni netra, his fiery eye, Shivji can burn, but he will burn kaam, lust, those things unfavorable on the worshippers path. Even with His fiery aspect the snow clad mountain cools His body.
• Ekant: Shivji loves to stay in solitude; though he is recognised as Prajanath or Pashupatinath, but his ruchi and his swabhav are ekant, He prefers to reside in solitude. In solitude He meditates upon His Lord.