Praise be to Allah.
With regard to what is obligatory, Hajj is only required once in a lifetime. It was narrated that Abu Hurayrah said: The Messenger of Allaah (peace and blessings of Allaah be upon him) addressed us and said: “O people, Allaah has enjoined Hajj on you, so perform Hajj.” A man said: “Is it every year, O Messenger of Allaah?” He remained silent until (the man) said it three times. Then the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “If I said yes it would be obligatory and you would not be able to do it.” Then he said: “Be content what I leave with you, for those who came before you were destroyed because they asked too many questions and argued with their Prophets. If I tell you to do something then do as much of it as you can, and if I tell you not to do something then abstain from it.” Narrated by Muslim, 1337.
It was narrated from Ibn ‘Abbaas that al-Aqra’ ibn Haabis asked the Prophet (peace and blessings of Allaah be upon him): “O Messenger of Allaah, is Hajj every year or just once?” He said: “It is only once, and whoever does more than that, it is a voluntary (act of worship).” Narrated by Abu Dawood, 1721; classed as saheeh by al-Albaani.
With regard to what is preferable, the more a Muslim does of Hajj, the better, even if he can do Hajj every year. There are reports which encourage doing Hajj a great deal, such as the following:
1 – It was narrated from Abu Hurayrah that the Messenger of Allaah (peace and blessings of Allaah be upon him) was asked: which deed is best? He said: “Believing in Allaah and His Messenger.” He was asked: Then what? He said: “Jihad for the sake of Allaah.” He was asked: Then what? He said: “An accepted Hajj.” Narrated by al-Bukhaari, 26; Muslim, 83.
2 – It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “Whoever does Hajj for the sake of Allaah, and does not utter any obscenity or commit any immoral action, will go back (sinless) as on the day his mother bore him.” (al-Bukhaari, 1449; Muslim, 1350).
3 – It was narrated that ‘Abd-Allaah ibn Mas’ood said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Make the Hajj and ‘Umrah follow each other closely, for they remove poverty and sins as the bellows removes the dross of iron, gold and silver, and an accepted Hajj brings no less a reward than Paradise.”
Narrated by al-Tirmidhi, 810; al-Nasaa’i, 2631. Classed as saheeh by Shaykh al-Albaani in al-Silsilah al-Saheehah, 1200.
And Allaah knows best.