The Congressional District [CD] method is by far the fairest. It is based upon how we elect our Representatives and Senators. One vote for the winner of each specific district and two votes for the winner of the overall popular vote within the state.
Using the current method, there are normally solid minority-party districts within a solidly Blue [or Red] state. Minority-party voters go to the polls and their candidate wins the district - only to have their vote REVERSED and given to the winner of the overall winner of the popular vote. And for this reason, MANY minority-party voters simply DO NOT vote.
The CD method would ENCOURAGE voters of the minority party within a solidly Blue [or Red] state to show up at the polls since they know that their vote WOULD actually count at the district level.
Additionally, many MAJORITY-PARTY voters stay away from the polls in a solidly Blue [or Red] state, since the overall winner of the popular vote is a foregone conclusion.
The CD method would ENCOURAGE MORE voters of the MAJORITY-PARTY within a solidly Blue [or Red] state to show up at the polls since they know that their candidate is at risk for the electoral vote assigned to their district.
With NPV, the possiblity of NATION-WIDE recounts is VERY real - in razor-thin elections, since EACH AND EVERY VOTE counts in order to determine the winner. This could FORCE EVERY STATE [including ones NOT participating in the NPV] into a MANDATORY recount - even if a particular state has a LANDSLIDE for one candidate.
The CD method ELIMINATES this possibility by RESTRICTING recounts to the districts [and sometimes states]. Districts with landslide victories WOULD NOT be required to recount in order to determine the awarding of the one electoral vote - only districts with razor-thin popular vote margins. Same holds true at the state level - only those with razor-thin overall popular vote margins would recount for the extra two electoral votes.
The NPV may also be unconstitutional under the Interstate Compact Clause and/or the 14th Amendment of the Constitution.
The Supreme Court has ruled that:
"The individual citizen has no federal constitutional right to vote for electors for the President of the United States unless and until the state legislature chooses a statewide election as the means to implement its power to appoint members of the Electoral College. U.S. Const., Art. II, §1. This is the source for the statement in McPherson v. Blacker, 146 U.S. 1, 35 (1892), that the State legislature’s power to select the manner for appointing electors is plenary; it may, if it so chooses, select the electors itself, which indeed was the manner used by State legislatures in several States for many years after the Framing of our Constitution."
BUT HAS ALSO RULED THAT:
"When the state legislature vests the right to vote for President in its people, the right to vote as the legislature has prescribed is fundamental; and one source of its fundamental nature lies in the equal weight accorded to each vote and the equal dignity owed to each voter."
AND THAT:
"The right to vote is protected in more than the initial allocation of the franchise. Equal protection applies as well to the manner of its exercise. Having once granted the right to vote on equal terms, the State may not, by later arbitrary and disparate treatment, value one person's vote over that of another. See, e.g., Harper v. Virginia Bd. of Elections, 383 U.S. 663, 665 (1966) (“[O]nce the franchise is granted to the electorate, lines may not be drawn which are inconsistent with the Equal Protection Clause of the Fourteenth Amendment”). It must be remembered that “the right of suffrage can be denied by a debasement or dilution of the weight of a citizen’s vote just as effectively as by wholly prohibiting the free exercise of the franchise.” Reynolds v. Sims, 377 U.S. 533, 555 (1964)."
Additionally, the NPV may run into a problem with an obsure passage in the 14th Amendment:
"But when the right to vote at any election for the choice of electors for President and Vice-President of the United States, Representatives in Congress, the Executive and Judicial officers of a State, or the members of the Legislature thereof, is denied to any of the male inhabitants of such State, being twenty-one years of age, and citizens of the United States, or in any way abridged, except for participation in rebellion, or other crime, the basis of representation therein shall be reduced in the proportion which the number of such male citizens shall bear to the whole number of male citizens twenty-one years of age in such State."
The KEY phrases here are:
"But when the right to vote at any election for the choice of electors for President and Vice-President of the United States ... is denied ... or in any way abridged ..."
The NPV ESSENTIALY does just that. The right to vote for the CHOICE of electors is granted by the state. And the choice within the state will stand - AS LONG AS THE CHOICE WINS THE OVERALL POPULAR VOTE. IF the choice IS NOT the overall winner of the national popular vote, the choice is then SUBSTITUTED by the state and given to the winner of the NPV ...
I would award the two at-large votes to the winner of the majority of the state's CDs (or split the at-large votes in the event of a CD tie).
That helps to firewall vote fraud. E.g., if the at-large votes are based on the CD results, then stuffing the ballot box only wins the districts where it happens, as opposed to possibly tipping the state's popular vote, thus stealing the two at-large votes as well. It also means that, if the results are challenged, you only need recount the disputed districts, not the whole state (or the whole country!).