Impeccable, faultless, flawless, errorless are comparable when they mean absolutely correct and beyond criticism.
Impeccable usually applies to something with which no fault can be found or which is irreproachably correct.
Faultless is often used in place of impeccable without loss, but it is sometimes preferred when the emphasis is upon the absence of defect or blemish rather than upon technical correctness.
Its distinctive implication, however, is often that of insipidity or tediousness.
Flawless applies especially to natural products in which no cracks, blemishes, or imperfections can be detected or to character or reputation which is admirably excellent or to a work of art or its execution when comparably fine.
Errorless usually implies absence of all mistakes, especially of such mistakes as are technically regarded as errors; thus, an errorless baseball game may not involve flawless playing.