Perhaps in love is the wrong word when it comes to most of the girls, but I think all of them have shown attraction to him at least once during the show.

Eva- during the special, mostly, but also in his introduction...
Katie- Do you even have to ask? She and Sadie were his official fangirls for a while.
Sadie- Ditto, he even managed to make them fight...
Izzy- Well, according to her, they used to date, and she gave him the case...
Beth- Yep, we see some in TDI but mostly in TDA...
Courtney- Not much from her, but she was the first girl to be awed by him...
Bridgette- Hmm, I don't really remember seeing her fawn over Justin...maybe in the first episode or the talent show?
Lindsay- In TDA, you see it alot, despite the fact she is still thinking about Tyler.
Heather- in the talent show episode for a bit, and I think in the first, but suprisingly not alot from her.
Gwen- In the special, she told Justin about where not to look for the case despite Trent being a few steps away.
LeShawna- in the special, alongside Gwen, and a bit in TDA.

the ones who are least affected by him are Heather, Courtney, Bridgette and Izzy is just crazy lol

Izzy actually had an interest with him. Remember the TDI special when she had the case and she was confronted by Justin? Justin took off his shirt and she was blushing when she gave the case to him. And also the fact that she was dating him during her time off in TDI.

Yea, Izzy was holding the case, got tricked by Justin, handed him the case and then broke out of her trance and said, "where's the case?!"

And Noah, from the ground, said, "you gave it to a male model!"

Haha, yea, the least affected are Bridgette, Courtney, and Heather. Or at least, the ones who he hasn't really tried to charm.

I'd Say Heather is the least
followed by Bridgette
then Courtney

Heather is effected by him though, her evilness just superseeds it