Abu Bakr As-Siddique was the closest of the Companions in relationship to the Prophet (sas). From Sahih Al-Bukhari: Ibn Abbas reports: "The Prophet (sas) came out during his illness from which he died his head bound with a cloth. He sat on the minbar, thanked Allah, praised Him and said: "There is no one among the people who has been more generous to me with his life and his property than Abu Bakr ibn Abi Quhaafa and if I was to take a bosom friend, I would take Abu Bakr as my bosom friend. But, the friendship of Islam is better. Block off every door in this Masjid except the door of Abu Bakr."
Being the closest Companion to the Prophet (sas), Abu Bakr was also the most knowledgeable of them and the quickest to both believe and to understand teachings of the Prophet (sas), as we see in the following hadith:
Abu Said Al-Khudhri reports: The Prophet (sas) gave a khutba and said: "Allah gave a slave the choice between this world and that which is with Him and that slave chose that which is with Allah." Abu Bakr began to weep and we were surprised that he should cry like that just because the Prophet (sas) mentioned that Allah gave a slave a choice. As it turned out, the Prophet (sas) was the one who was givent the choice and Abu Bakr was the most knowledgeable among us. Sahih Al-Bukhari
Shortly before his death, the Prophet (sas) gave us very important wasaaya or "parting advice" on a variety of subjects. Among them were the virtures of Abu Bakr. Also among them were various warnings about practices of shirk and bid'a which the nations have alway fallen into in periods if ignorance after the messenger has departed. One such practice was excess with regard to graves of prophets and righteous individuals. Turning graves into places of worship (or burying people in places of worship) is completely forbidden in Islam and has been one of the main gateways to shirk through the ages.
From Sahih Muslim, Jundub narrates: I heard the Prophet (sas) five days before his death saying: "I declare my innocence before Allah that I should have any bosom friend among you for Allah Most High has taken me as His friend just as He took Ibrahim as His friend. If I were to take any bosom friend from my nation I would take Abu Bakr. Listen! Those who came before you took the graves of their prophets and their righteous ones as places of worship. Listen! Do not take graves as places of worship. I forbid you that."
How sad it is that so many of our Muslim brothers and sisters continue to live in disobedience and disrespect of this great wasiya (final advice) of the Prophet (sas).
From Sahin Al-Bukhari Amr ibn Al-Aas narrates: The Prophet (sas) sent me to the army during the battle of Dhat Al-Salaasil. When I returned to him (sas), I asked him (sas): Who is most beloved to you? He said: "Aisha." I said: What about among men? He said: "Her father." I said: And then who? He said: "Then Umar ibn Al-Khattab." And then he listed some other men.
The Companions were well aware of who were the first few Companions in closeness to the Prophet (sas) and the Rightly-Guided Khalifas were chosen on that basis. From Sahih Al-Bukhari, Ibn Umar reports: We used to discuss the best of the people during the time of the Prophet (sas) and we saw the best as Abu Bakr, then Umar, then Uthman ibn Affaan, may Allah be pleased with them all.
Ali ibn Abi Talib (ra) was also very much aware of this and bore witness to it as we see in this narration from his son which is found in Sahih Al-Bukhari also: I said to my father: Who is the best of the people after Allah's Messenger (sas)? He said: "Abu Bakr". I said: And then who? He said: And then Umar ibn Al-Khattab. I was afraid that he would say Uthman next, so I said: And then you? To which he answered: I am nothing but a man among the Muslims."
Also, from Sahih Al-Bukhari: Once, there was a problem between Abu Bakr and Umar. Eventually, Umar came to the Prophet (sas) and said: O Allah's Messenger, I was wrong. He repeated it twice. Then, the Prophet (sas) said: "Allah sent me to you and you said: 'You lie.' but Abu Bakr said: 'He speaks the truth.' And he supported me with his life and his property so could you please leave for me my companion?" He said it twice. Abu Bakr was never mistreated after that.
Abu Bakr was the quickest of the Companions to rush to any good deed. From Abu Daud (with an authenticity of "hassan"): Umar ibn Al-Khattab said: The Prophet (sas) ordered us to give sadaqa. It happened to coincide with some wealth I had just acquired and I said to myself: If I will every surpass Abu Bakr, this is the day I will surpass him. And so I cam to the Prophet (sas) with half of my wealth and he (sas) said to me: "What have you left for your family?" I said: The same amount. Then Abu Bakr came with all of his wealth. The Prophet (sas) said to him: "What have you left for your family?" He said: I have left them Allah and His Messenger." I said: I will never surpass you in anything."