What are the necessary ingredients of a good solo(ish) superhero?

[DrNewGod]

First: there's no way any one poll on this is going to be all-inclusive of what it takes, so feel free to comment and maybe subsequent polls can revise and narrow the list. When the thread title says "solo(ish)" it doesn't preclude a character joining a team, even early on (e.g. Black Panther or Booster Gold) but focuses on those heroes created as solo entities, even if only in the internal logic of the story (Iceman or Negative Man probably wouldn't qualify). It also assumes the character is meant to carry a title, or at least a feature in a title, if only eventually so.

Aside from a good writer and good plots, what does it take?

[Zelena]

A moral conundrum and rich, complex characters…

[the illustrious mr. kenway]

Their strengths and flaws as a person than their powers. A good supporting cast and status quo also help. The supporting cast includes the antagonists too.

Powers/physical skills do matter but they are more like a tool to use. They provide spectacle which could draw people in but not always why they stick around.

I'm a bigger fan of Daredevil because he has a more interesting character/conflicts/supporting cast while Iron Fist has cool powers but lacks interesting characteristics/conflicts etc.

[Catlady in training]

Solid supporting cast.

[Mister Mets]

This is a fair question.

Some stuff is typically good, but not essential. So it may be possible to have a superhero without a love interest, even if I like plenty of stories about heroes happily or unhappily in love.

I think the most important thing is a story engine to get the superhero into situations where he or she is needed, and where there's the potential for crazy conflict. Peter Parker and Clark Kent are journalists, with superpowers that can inform them of things that are suspicious. Tony Stark and Bruce Wayne run major corporations, so they have the resources to hunt down crime, and may also respond to it when it affects their companies. Bruce Banner is often a fugitive. Barry Allen is a police investigator. Matt Murdoch is a lawyer whose speciality intersects with organized crime. Thor and Wonder Woman are essentially ambassadors of fantasy kingdoms.

Now, looking at the choices.
Compelling Motive: This does seem important.
Hand-to-Hand Combat Skills: You can have a superhero without this.
Transportation Abilities/Equipment: I don't think you can have a superhero without some method of getting to dangerous situations quickly.
Injury Resistance: Helpful, but not essential.
Ranged Attack Abilities: Helpful, but not essential.
Interesting Costume: Important, but not absolutely necessary.
Secret Identity: Iron Man showed that it's not necessary.
Extensive Supporting Cast: Helpful, especially if a character is prolific, but there are some decent runs that focus on a small cast.
Superhuman Athletic Ability (or tech): This is kind of what distinguishes superheroes.
Extensive Roster of "Proprietary" Antagonists: There are good stories/ runs for superheroes with lame rogues galleries. That said, it can help. Green Lantern didn't need the villains Geoff Johns introduced/ revamped, but it improved the series.
Well-Depicted Diversity: This is typically important, but not absolutely necessary. Does a Black Panther run need to have multiple characters who aren't Wakandan?
A Sense of Optimism: Cynical heroes can work.
A Sense of Cynicism: Optimistic heroes can work.
Coherent Character+Abilities Theme: A superhero won't be distinctive without it.
Love Interests: Not absolutely necessary.

[DrNewGod]

This is a fair question.

Some stuff is typically good, but not essential. So it may be possible to have a superhero without a love interest, even if I like plenty of stories about heroes happily or unhappily in love.

I think the most important thing is a story engine to get the superhero into situations where he or she is needed, and where there's the potential for crazy conflict. Peter Parker and Clark Kent are journalists, with superpowers that can inform them of things that are suspicious. Tony Stark and Bruce Wayne run major corporations, so they have the resources to hunt down crime, and may also respond to it when it affects their companies. Bruce Banner is often a fugitive. Barry Allen is a police investigator. Matt Murdoch is a lawyer whose speciality intersects with organized crime. Thor and Wonder Woman are essentially ambassadors of fantasy kingdoms.

Now, looking at the choices.
Compelling Motive: This does seem important.
Hand-to-Hand Combat Skills: You can have a superhero without this.
Transportation Abilities/Equipment: I don't think you can have a superhero without some method of getting to dangerous situations quickly.
Injury Resistance: Helpful, but not essential.
Ranged Attack Abilities: Helpful, but not essential.
Interesting Costume: Important, but not absolutely necessary.
Secret Identity: Iron Man showed that it's not necessary.
Extensive Supporting Cast: Helpful, especially if a character is prolific, but there are some decent runs that focus on a small cast.
Superhuman Athletic Ability (or tech): This is kind of what distinguishes superheroes.
Extensive Roster of "Proprietary" Antagonists: There are good stories/ runs for superheroes with lame rogues galleries. That said, it can help. Green Lantern didn't need the villains Geoff Johns introduced/ revamped, but it improved the series.
Well-Depicted Diversity: This is typically important, but not absolutely necessary. Does a Black Panther run need to have multiple characters who aren't Wakandan?
A Sense of Optimism: Cynical heroes can work.
A Sense of Cynicism: Optimistic heroes can work.
Coherent Character+Abilities Theme: A superhero won't be distinctive without it.
Love Interests: Not absolutely necessary.

Well considered analysis

[DrNewGod]

Another thing strikes me is that the costume needs the be relatively simple. Not just to provide an effective brand image, but so it can be quickly rendered by a variety of illustrators.

[the illustrious mr. kenway]

Legacy characters overlap with solo heroes. They just need a clearer connection with the source hero but also need new angles to explore. Flash Thompson's time as Agent Venom is a good example of this.