Gemara

(R. Yochanan): Nevi'im prophesized about the reward awaiting penitent sinners, but the reward of Tzadikim who never sinned "Ayin Lo Ra'asah..."

He argues with R. Avahu.

(R. Avahu): Tzadikim who never sinned cannot stand where Ba'alei Teshuvah stand (the latter have a greater share in the world to come). "Shalom Shalom la'Rachok vela'Karov" - the Rachok (far) is in front of the near;

'Far' refers to one who was far from Hash-m (but repented). 'Near' refers to one who was always close.

R. Yochanan explains that 'far' is one who distanced himself from sin. 'Near' is one who was close to sin and distanced himself.

Kidushin 49b: If one was Mekadesh 'on condition that I am a Tzadik', even if he is (established to be) an absolute Rasha, she is (Safek) Mekudeshes. Perhaps he repented in his heart.

Rishonim

Rambam (Hilchos Teshuvah 7:4): Tzadikim who never sinned cannot stand where Ba'alei Teshuvah stand. A Ba'al Teshuvah is greater, for he overcomes his nature more than a Tzadik who never sinned.

Ohr Some'ach: The Rambam learns from Sotah 36b that a Ba'al Teshuvah is greater. It says that Yosef was Mekadesh Shem Shamayim in private, and a letter of Hash-m's name was added to his name. Yehudah was Mekadesh Shem Shamayim in public, so his entire name was based on Hash-m's name.

Rada (in Likutim in Frankel Rambam): The Rambam discusses the ideal Teshuvah, i.e. he was still a youth and was tempted by the same woman... Surely, after a standard Teshuvah he is not greater than one who never sinned.

Maharsha (Berachos 34b DH she'Ba'alei): It is difficult to say that R. Avahu considers a Ba'al Teshuvah is greater than one who never sinned. Rather, he discusses one who was close to sin, i.e. intended to sin, and overcame his Yetzer Ra. R. Yochanan holds that one who never sinned even through thought is greater.

Yabi'a Omer (3 EH 8): The Gemara and Poskim say that if a Rasha was Mekadesh 'on condition that I am a Tzadik', she is (Safek) Mekudeshes. Kol Eliyahu (EH 2) was asked about one who said 'on condition that I am a total Tzadik.' He brought from Chasdei David that since Yom Kipur does not atone until the day ends, she is not Mekudeshes until the end of Yom Kipur. Kol Eliyahu adds that if he transgressed Chayvei Kerisus or Misos Beis Din, since their Kaparah is only after afflictions come, she is not Mekudeshes until he gets afflictions. If he transgressed Chilul Hash-m, for which Kaparah is only after death, she is not Mekudeshes. However, the Gemara often says 'total Tzadik' even if he is not a total Tzadik, e.g. one who says 'this coin is for Tzedakah in order that my son will live' (Rosh Hashanah 4a). The Rambam rules like R. Avahu against R. Yochanan, that a Ba'al Teshuvah is greater than a Tzadik who never sinned. The Ragatz'over says that he learns from Kidushin. If a Ba'al Teshuvah were less than a total Tzadik, she would not be Mekudeshes! Rather, this shows that a Ba'al Teshuvah is no less than a total Tzadik. This is unlike Chasdei David, who says that a Ba'al Teshuvah is not a total Tzadik (until Motza'ei Yom Kipur). The Minchas Chinuch (364) says that when one repents due to love, his Aveiros become Mitzvos without Yom Kipur and afflictions. We should be concerned lest he did so, and already he is a total Tzadik! We can say that Teshuvah from love is rare and we are not concerned for it. However, we could say that the Rambam discusses Teshuvah from love, and R. Yochanan discussed Teshuvah from fear. Several Poskim cite the Gemara to say that even if he stipulated 'that I am a total Tzadik', and he had transgressed Chayvei Kerisus and Misos Beis Din, she is Mekudeshes. The Or Zaru'a says that even if he transgressed Kerisus and Misos, once he repents in his heart he is a total Tzadik in every respect, just he must afflict himself to atone for his Aveiros. Also the Bach (Teshuvah 102) and many others say that once he repented in his heart, he is a total Tzadik.

Hagahos Ashri (Chulin 7:16): If one claimed to sell Kosher meat and really it was Tereifah, it is forbidden to bury him, even if dogs are eating his flesh and licking his blood. If he repented, (we bury him, for) nothing blocks Teshuvah.

Poskim

Rema (OC 581:1): For Slichos and Yomim Nora'im, we are meticulous to get the most proper Shali'ach Tzibur and the greatest in Torah and deeds.

Teshuvas Mizrachi (88): The Rambam (Hilchos Tefilah Perek 8) lists people who may not be Shali'ach Tzibur. He does not list one who sinned and repented.

Mishnah Berurah (11): He should be a total Ba'al Teshuvah. One who went to Nochri courts should not be Shali'ach Tzibur on Rosh Hashanah or Yom Kipur unless he repented.

Shulchan Aruch (YD 362:5): We do not bury a Kosher mediocre Tzadik next to an outstanding Chasid.

Beis Yosef (DH u'Mah she'Chosav Ein); When Rav Huna died, they took the body to Eretz Yisrael. They suggested burying him next to R. Chiya, for both of them spread Torah. A Talmid (who never requested permission of his Niduy) was brought into the cave of a Chasid, but he was not accepted there. He was brought into the cave of judges, and he was accepted there, for he did like R. Ilai (if one cannot overcome his Yetzer, he goes elsewhere so he will not be Mechalel Hash-m (Mo'ed Katan 17a, 25a).

Prishah (14): This is because they will not have serenity together.

Gilyon Maharsha: (Even regarding a Rasha next to a Tzadik) it is only l'Chatchilah not to bury them next to each other.

Rema: We do bury a Ba'al Teshuvah next to a total Tzadik.

Darchei Moshe (1): This is from Or Zaru'a.

Bach (DH Kosav, cited in Shach 6): Or Zaru'a says that a Tzadik is one who fulfills the laws of the Torah. A Chasid is Mekadesh himself through (refraining from) what is permitted. We may bury a Ba'al Teshuvah next to a Tzadik. The Or Zaru'a connotes that we do not bury a Ba'al Teshuvah next to a Chasid.

Gra (9): See the end of Perek 5 of Berachos (which is the same as our Gemara).

Shevet ha'Levi (9:248): Agra d'Pirka expounds "Ozen Shoma'as Tochachas Musar b'Kerev Chachamim Yalin" based on the Ari Zal, who says that we do not bury a Rasha next to a Tzadik because a Ru'ach comes to reveal secrets to the Tzadik, unless a Rasha is nearby who may not hear them. Even if one was not a Ben Torah, if his ear accepted rebuke, his ear will be rewarded and it may hear the secrets, so one may bury him next to a Tzadik. Minchas Eliezer struggled to resolve this with the Halachah. I could explain the verse like the simple reading of the Rema (unlike the Bach), that a Rasha who accepted rebuke and repented may be buried even next to a Chasid.

Divrei Yatziv (YD 66 DH ul'Fi): I say that 'Tzadikim who never sinned cannot stand where Ba'alei Teshuvah stand' does not refer to real sinners. Hash-m told Reuven 'you were the first to repent. Your descendant (Hoshe'a) will open the door for Yisrael to repent - "Shuvah Yisrael" (Bereishis Rabah 4:18). Adam ha'Rishon and others already repented! Rather, Reuven did not really sin (Shabbos 55b). Based on his level, it is as if he sinned to Hash-m. He was the first to repent from such a matter.

Yechaveh Da'as (5:9): Most say that a Chasid is greater than a Tzadik. However, R. Yehudah bar Yakir says oppositely. He says that Chasid refers to a Ba'al Teshuvah. A Tzadik cannot stand where a Ba'al Teshuvah stands, for a Tzadik is like a noble in front of the King, and a Ba'al Teshuvah is like a slave, who enters where the noble may not (Berachos 34b). Therefore, in Tefilah we request for 'Al ha'Tzadikim', who are greater, and afterwards 'v'Al ha'Chasidim', who are lower. The Avudraham says similarly. David said "Chasid Ani", for he was a Ba'al Teshuvah. He could not say that he was a Chasid (surely this is a printing mistake, and it should say 'Tzadik' - PF). Sukah 53a connotes otherwise. At Simchas Beis ha'Shoe'vah, Chasidim said 'our youth did not shame our old age'. Ba'alei Teshuvah said 'our old age atoned for our youth.' However, Bava Kama 103b brings an episode with a Chasid. It says 'do not say that he swore falsely and repented, for every episode with a Chasid refers to R. Yehudah.' This implies that one who sinned and repented could be called a Chasid.