The most important right a defendant has in plea negotiations is his fifth amendment right not to be a witness against himself. If/when he enters a plea, he has waived that right. The above post emphasizes the importance of sixth amendment rights, which is also important; but to me is secondary to a defendant's right to remain silent. He does have the right to counsel, but most guilty pleas are negotiated with the benefit of counsel. Before accepting a guilty plea, the defendant is normally asked by the presiding judge if he is happy with the representation he has received from his counsel. It should also be noted that by entering a plea of guilty, a defendant has waived his right to a trial by a jury of his peers. Defendants are normally reminded of this by the presiding judge so that the court may be satisfied that the plea is entered into voluntarily, freely, and with full knowledge of the rights available.
I think that the defendant's sixth amendment rights are strongest attached to the concept of a plea negotiation. It is here where the criminal procedures post- arrest are laid out. Part of the implication of the "speedy trial" is the idea that the defendant is able to see in a "quick" manner the evidence laid out on the charge being made. If the prosecution lacks the evidence for the higher charge, the plea bargain process happens here in that negotiations take place to see that some aspect of justice is done for both parties. The Sixth Amendment's need for the defendant to be aware of the chage leveled against them also is triggered here. The presence of counsel for the defendant is also addressed in the Sixth Amendment and is a part of how the defendant is able to experience a check on the prosecution. In the end, while the judge can revoke a plea agreement and while the prosecution can turn back on it, the idea is that if an agreement has been reached in good faith, all parties end up benefiting. The prosecution is able to show some justice being done, the defendant has not had his/ her rights violated in the process, and the judge has been able to sanction the process taking place. It is in this where the Sixth Amendment's protection of the plea bargain process is strongest.