This tier list differs a bit from the multiplayer list because, for single player, the most important thing is the value your character brings to yourself. Having tools for your team matters a lot less because you don't have any besides your robot friend.
Retcon
This is the biggest change compared to multiplayer. While Retcon is by far the weakest multiplayer character, she is one of — if not the best — single-player characters.
Her abilities only benefit herself, which is incredibly valuable in single-player, making her completely independent from teammates.
She is extremely tanky because of her Rewind Time ability, allowing her to reverse any damage taken as long as the ability is used in time.
She already synergizes well with damage upgrades, but her Rage ability makes it even stronger, increasing her damage by 100% for 15 seconds.
Last but certainly not least, the first time you go down during a mission, you receive a free revive with full HP and armor. No other character has access to this ability, nor are there upgrades that grant it, making her truly unique.
Spotter
Spotter is insane in multiplayer, but his value is just as high in single-player. He can easily detect anything on the map thanks to his Scanner. This is especially useful for finding enemies on ceilings, such as Cave Leeches.
His Crit Darts are just as strong because he can obviously use them himself to great effect. They still effectively double his damage until you either gain enough crit chance to stop relying on them or continue building around them entirely.
Lastly, his Ult is extremely useful since it gives you 50% free ammo whenever you need it. This is massive during boss fights, events, or elevator sequences.
Falconer
Just like in multiplayer, Falconer is still in S tier. She brings massive value to single-player with all the elemental damage bonuses she can apply to herself.
Her revive is, of course, not that useful in single-player since there are no teammates to revive. Besides that, however, her damage output is completely insane.
She can utilize elemental damage better than any other character and has great synergy with upgrades that use electric or shock damage.