The iconics are intentionally not optimized. Many are anti-optimized, picking badly mismatched styles or bad weapons. What do you think are the strongest despite this?
Sorcerer and Wizard have relatively strong spell selection.
Bloodrager's use of Furious Focus is questionable, but otherwise fairly reasonable.
Warpriest has poor spell selection at higher levels, but otherwise competent.
Kineticist is pretty terrible due to being all fire damage but at first level, Yoon will never encounter significant amounts of fire resistance.
Barbarian: The improperly sized sword is not worth it.
Cleric: Healing domain is a bad choice, and choosing that for a domain spell is bad when you can convert anything else to them anyways.
Fighter: TWF without both weapons gaining weapon training and no gloves of dueling is poor.
Monk: No-archetype chained Monk.
So I think it depends what you want from the Iconics. These characters aren't designed to be super optimised for the number crunch, they're designed to teach new players how to play. This really changes how you look at what "optimised" means.
Amiri the Barbarian has some amazing flavour, and believe it or not her damage output is actually pretty good. At low levels she has a fairly low chance to hit (50-60% at level one vs average enemies), but she's likely to 1-shot almost anything she comes across even without Power Attack or Rage. She's the "Iconic" barbarian - she's reckless and unpredictable, but you really don't want to get hit by that sword.
Kyra the Cleric isn't optimised for combat, but she's got some decent buffing capabilities. She cast spells, she can channel and she can buff herself and mix it up in melee if needed. She's built to show off a few aspects of the class. The healing spells are included so that new players know what they do. The Healing domain isn't the strongest, but healing spells are a way to buff the party AFTER a mistake has been made (something very useful in a party of first-time players). Kyra's empowered cure spells make the in-combat healing useful enough to actually be worth an action. Sure Cure Light Wounds isn't going to cut it at for long, but when I last played a level 7 Kyra I cast Empowered Cure Critical Wounds (~37hp) on Amiri to keep her alive long enough to end the encounter. And if you're in an undead heavy module Kyra becomes a pretty serious damage dealer as well.
I don't have enough experience with most of them to comment, but those two at least are better than they get credit for. You have to be a bit clever about how you use them, but they're no what I'd call "anti-optimised".
I think the Harsk Ranger is probably the worst, simply because he uses a crossbow. The reason he uses it is that they comissioned the art long before they thought of giving the iconics stats. This is also why Valeros uses less than optimal weaponry. I don't know for sure, but I assume Valeros was also written before Gloves of Duelling were a thing (even if not, I only have stats for the Iconics uo to 7th level, and blowing 15,000gp on a set of gloves would be pretty crazy at that level).
I only played a few games of PFS and it was at a con so they gave me an iconic. I played the paladin
The thing that unnerved me was she has a trait to make survival a class skill and had 0 ranks in it.
That being said, I think most agree harsk is the weakest.
Spell users have so little that matters besides spells I guess by default they are the strongest?
I'm not too familiar with the iconics in Pathfinder, but I recall the ones in 3.0. Lidda (the halfling rogue) reached 15th-level and still had not taken Weapon Finesse. Designers really should understand the basics of game balance and optimization :(
There was probably nothing that could be done to make the chained monk worthwhile using just core rules, in either edition.
(Meant to say Slayer is TWF. Though this is only sub-optimal at level 1 since Slayer's don't get SA till level 2)
Problem with Amiri/Barbarian is that going from 2d6 to 2d8 is a mere 2 damage. With the to-hit penalty, she's spending a feat to make herself worse. At level 1, average damage from a greatsword (13) is sufficient to kill most things before power attack or rage, so she doesn't need the extra damage.
As for Feiya/Witch, she looks usable. Not sure why she wasted money on a +2 quarterstaff, and her spells prepared could be improved (Can you change an iconic's prepared spells in PFS without resting mid mission?), but as long as they have half-way competent spell-selection and good primary attribute, full casters are hard to mess up.
I've GMd for and played many of the iconics over the years. How the player plays the iconic can make a big difference. For example, iconic Kyra, the cleric, becomes much more formidable when played by an experienced player and able to rest, change out spell load, and acquire a 5gp longspear. Most PFS scenarios provide ZERO chance to change prepared spells or acquire a simple 5 gp weapon, but some do. E.g. The 7th level Kyra, if given rest time to change prepared spells, can load up on Summon Monster and Open Spell Slots, leading to a much more effective character.