The Two Most Naieve
I'd have to say the fat video-game player and the Australian.
These guys need some self-awareness. I felt sorry for the fat guy. He ended up chasing probably the hottest girl on the show. What was he seriously thinking? I can KIND of understand the idea that a "poor, Ukrainian woman" would look at him as rich, and that enough would get her to the USA. But do you really think she'd stay with you? Once back in the USA, you'd continue to get fat, and you'd be playing video games all the time. She'd meet someone much better looking and dump you. Gotta be more self-aware bro; try and marry someone more your type.
The Australian guy was just as naieve. I could tell the way she looked at him (or didn't) that she wasn't interested. Every time he hugged her she almost looked repulsed. And every time he tried kissing her, she'd turn her cheek just a tad so that she didn't have to kiss him on the mouth.
The one that kind of surprised me was the Wisconsin dairy farmer. It sure looked like he and his girl had something genuine. But as soon as he was taken for some money, I knew he'd been scammed (as did he)
Kind of felt sorry for some of these guys. Love doesn't come easy. You can't sit around on your ass, never work out, and play video games all the time and think that a super-hot woman is going to want you.