This is a good question! I think the answer varies depending on if we’re talking about who taught Harry the most useful skills vs who taught the general student body the most useful skills, and if said teaching has to be limited to when these professors were officially in the DADA role.
If we’re going by skills Harry learns that are useful to his mission, I think the answer would have to be either Lupin (Patronus charm, and possibly some secondhand teaching via the defense book Lupin gifts him in OOTP) or Snape (bezoars, expelliarmus and muffliato—which gets a fair amount of use in book 6 and as part of their protections in 7). Crouch as Moody did teach Harry the unforgivables and how to throw off the Imperius curse, which is definitely worth mentioning, but Harry only used those skills successfully twice.
If we’re going by most valuable teacher for the entire student body, I think it would be either Moody or Snape, not Lupin. Lupin’s lessons on the Patronus charm, while helpful to Harry, were not taught to the entire class. For every other student, Lupin’s lessons were centered around Dark Creatures, which were of limited use.
Although some of Snape’s lessons were private (Occlumency, HBP book spells), most were taught for the benefit of the whole class. For example, Snape taught about antidotes and bezoars in his first and fourth year classes, and everyone at Dueling club was exposed to expelliarmus. As DADA teacher, his 6th yr classes included nonverbal spells and multiple approaches for fighting the effects of dementors, which would be helpful for those students who did not learn how to cast a spell as advanced as a Patronus like Harry did.
Crouch’s lessons were also accessible (perhaps alarmingly so, lol). He taught at least half the school (seems like 4th yr+) about unforgivable curses, demonstrated them, and even gave them ample practice under the effects.
Finally, if we are limiting this to only lessons taught while the professors were officially in the dark arts role, I would say Crouch/Moody. Some of Snape’s most useful lessons were taught in other capacities, and while his DADA lessons taught important skills, it seems like they were restricted to upper level students only, and we don’t get insight into what the other levels’ classes looked like.
Tho tbh I always thought Harry’s best DADA teacher was Hermione during the Triwizard tournament, as that’s when Harry learns a wide variety of combat-useful spells like the Impediment Jinx, Protego, Reducto, etc. I guess this opportunity was brought to him by Crouch Jr, so… another point to Crouch lol??
