I doubt it. He did OK in some respects, but had many other problems besides Watergate!
Are you asking if He had not been caught would he be the "best" president ever?
Hmmm. If Charlie Manson had not been caught, would he be the greatest guru ever?
If Charlie Whitman had not been killed, would he have been the greatest Marine ever?
If during WWII, had Germany won, would Hitler be the greatest chancellor of Germany ever?
You can get answers from no, to yes on all these. One cannot seperate lying, cheating, stealing, anti-social behaviour, killing, and the insatiable quest for personal power from history, and ask "what if", because it has no answer.
Richard Nixon went down because of his moral deficits, lying, cheating, and quest for absolute power. It was not because of a "scandal".
I cannot resist adding to Bob Pursley's excellent answer.
Nixon's "dirty tricks" squad, under the direction of his close friend and later convicted felon Atty. General John Mitchell, was involved in many other dubious activities besides Watergate, with Nixon's blessing. Just listen to the tapes of his oval office conversations if you want to know what kind of person Nixon was. On the other hand, his establishment of diplomatic relations with China, something that a Democratic president would have been afraid to do because of a fear of conservative public opinion, helped bring an end to the VietNam war and the cold war. He managed to choose a very fine successor to himself when he replaced his convicted corrupt vice president Agnew with Gerald Ford, who helped lead us out of the darkness.
Amen, DrWLS. Good details to include.