As @PhillyGuy said so well, it depends on what your goal is. It’s also unclear from your OP whether you mean to ask what’s best for an individual or what’s best for the entire electorate.
Assuming you meant what’s best for one individual person …
I’d rank them by convenience as:
- Mail first, early in-person voting second by a small margin, and in-person on Election Day dead last by a mile.
I’d rank them by likelihood of me screwing up and not having my vote count as:
- In-person on Election Day most likely, and the other two (mail and early in-person voting) a tie.
For primary elections, one of the risks of early voting (mail or in-person) is entering your vote then having your chosen candidate drop out before the primary election day. Balanced of course by the risk of you forgetting to actually vote before the deadline.
For general elections the risk of surprise is just about zero, so there’s no upside to waiting to near the deadline. Get it done ASAP.
I’d rank them by likelihood of somebody else screwing up and not having my vote count as:
- Mail most likely, early in-person voting second by a large margin, and in-person on Election Day last, but only by a small margin.
This ranking is just based on how many steps have to go right. Or said another way, how many opportunities for error there are and how much time passes between my last touching my ballot and the vote being registered for each method.
I’d rank them by likelihood of deliberate tampering preventing my vote from being counted as:
- Mail most likely, early in-person voting second but just barely second by a small margin, and in-person on Election Day last by a large margin.
Like with honest screw-ups, this ranking is mostly based on how many opportunities there are for skullduggery plus how many choke points permit mass-production skullduggery. Big difference between one angry mail carrier throwing away a few ballots they collected some days from neighborhoods likely to vote the “wrong” way in their opinion, versus some operatives being able to enter an election authority warehouse and examine and dispose of likely unfavorable ballots en masse at their leisure.
So which problem is the OP / us trying to solve?
As to me …
Prior to 2020 I never considered the security / skullduggery aspects at all. My sole considerations were convenience and avoiding the last chance single point of failure of me being unable to vote in person on Election day because of whatever random reason.
In 2020 I was somewhat concerned about ballot security / election rigging.
I’m about the same amount concerned in 2024 since it’s thousands of county-level local authorities, not the Biden administration, that are running these elections. Although the general public awareness of the likelihood of organized tampering is much higher than it was going into 2020, so that’s a mitigating factor.
If Biden wins and the Ds do well in Congress, the ballot security problem will be solved by 2028. If trump and the Rs win, the problem will be solved a different way: there won’t be 2028 elections.