According to Astro Vaastu, the recommended idols to be installed at the main gate depend on the ascendant sign:
For Aries and Scorpio ascendants, Lord Hanuman is considered auspicious.
For Taurus and Libra ascendants, a female deity is considered favorable.
For Gemini and Virgo ascendants, Lord Ganesh is believed to bring positive energy.
For Cancer ascendant, Lord Shreenath ji or Lord Shiva is recommended.
For Leo ascendant, Lord Surya (Sun god) is considered auspicious.
For Sagittarius and Pisces ascendants, Lord Vishnu or a statue of one's guru is preferred.
For Capricorn and Aquarius ascendants, Lord Shiva or a horse nail is recommended.
Additionally, it is suggested to have umro or chaukhat (doorframe) for the main gate, and the floor should be higher than the outside floor level. Furthermore, it is advised to paint auspicious signs on the floor of the main gate.
The above is based on lagna ( Ascendant) however planet sitting in Lagna or Aspecting the lagna will also play important role while suggesting the Perfect Idol.