Hit the nail right on the head with each girl and her relationship with Ash. Iris's interactions with Ash really just haven't been that...memorable in terms of how he interacted with May, Misty or Dawn.While I do think it's too early to be putting Iris in this sort of poll, I'm going to judge this based on how Ash interacted with the girls from the first 40ish episodes of their respective series. While it isn't much, usually by this point in any series there's a general idea how Ash bounces off with each girl. So that's the most logical way I'm gonna go about this. So far, I haven't seen anything where Ash's connection with the girls DRAMATICALLY changes as the series progresses, so I'll go on a limb and assume we won't be getting anything absolutely groundbreaking between Ash and Iris' "friendship" throughout BW's entirety.
With May, I felt a vibe of Ash being supportive and a source of inspiration/influence (though they displayed this with Dawn better). AG was the first time were Ash WASN'T in the role of "inexperienced newb" and it was a nice change of pace from what we had before. And while I never cared for Misty or her interactions with Ash, I can't deny that they were really good and close friends; the series usually does a good job about showcasing these tight friendships. Besides, Pokeshipping is powerful for a reason; I'd be flat-out in denial to say Ash and Misty never had memorable interactions. Their first meeting consisting of Ash stealing Misty's bike, frying it, then dragging her through a forest of bugs is pretty entertaining and grounds for the bickering that continued on.
With Dawn, they REALLY went out of their way to make perfect synergy with Ash. The high-touch, the influences they had on each other's battling and Contest styles (that were consistent and not one-offs). I guess arguably the influences weren't made obvious this early on in DP, but the high-touch definitely was. They got along, but they were also a lot like siblings to me. While they put a lot of work into Dawn and Ash's synergy, it was rather subtle during the early days of DP, but not to the point where it was totally unnoticeable. Either way, just like Misty and May, it was obvious that Dawn and Ash had a good, strong friendship by the 40-episode mark.
And Iris? Uh... yeah. Admittedly it's more unfair to judge Iris since the series is still going on, but I honestly feel nothing between her and Ash. She doesn't hesitate to snark at him, but I think she did it to the point of overload, which in turn reflected poorly on Iris' character. Or it did to me, anyway. But Ash usually pays no mind to her remarks (i.e. won't sink to her level) or makes unmemorable "What did you say/Whaddya mean by that/Oh, come on" responses and then the conversation is abruptly dropped. Really, there's nothing I find likable about Ash and Iris' interaction, and I don't feel the strong FRIENDSHIP vibe that I felt with him and the other girls 40 episodes into their respective series. To this day I still wonder why Iris wants or needs to travel in the group, and that's not a good sign. So while it may be too early to pass a completely fair and accurate judgment, I'm gonna say based on the first 40 episodes of every series that the weakest interaction with Ash is Iris, bar none.
LOL, i'm still wondering why she's traveling with Ash too.