This might take some thought, but I love Mac and should be able to whip some stuff out rather quickly here.
Yes, his crush on Frankie in "FMD" is absolutely adorible, and I too love that wobbly smile.
And I was pained in "A Lost Clause" when he's on Bloo's bed practically convulsing. Poor Mac, but yeah, that was cute too.
I love him in "I Only have Surprise" when he hugs Mr. H. Have we ever seen him hug Mr. H. before? I can't remember any other time... I also love the look on his face after he tackles Wilt and is looking up in bewilderment at the "Happy Earthday" banner.
I also love him at the end of "Infernal Slumber" when his mom yells his name just after he goes back to bed. He comes out of his room, clearly nervous, with big eyes, and says in a meek voice "Yeah?"
Speaking of "IS", I also love it when he goes bazurk at Bloo trying to do that shaving cream trick, especially the first time (with Terrence). He's talking so fast and trying to be quiet at the same time, we can't understand what on earth the boy is saying. Although after seeing the eppie a number of times, I think I can make out "Do you understand that when I die, you...." Or something like that.
Another cute scene which I think I just brought up yesterday is at the end of "Duchess of Wails" when he and Bloo failed at getting Duchess back in the house, they're covered in mud, and he thinks he's never going to see Bloo again. We see him initiate a hug (it's usually Bloo who hugs
him), and when he tells Frankie to forward his postcards, we can hear the emotion in his voice. Aww, I just want to hug him in that scene.
I also like him toward the end of "Partying", when Bloo covers him with a blanket and gives him sugar free gum. He starts gagging and choking and yelling "It burns us!", reminding me of Smiegal from LOTR. It's also cute when he's sitting between Wilt and Coco in the back of the car, still coughing and shivering, and Wilt has his arm around his back as if to protect him since he clearly isn't very competent at the moment.
In "Go Goo Go" after Mr. H. and Frankie accuse Goo of being his girlfriend, they're standing outside the door and Goo says "Ok, Mac, I'll let you hold my hand", and she grabs Mac's hand. But he just stands there like he just doesn't know what to do next, or even what to think about this situation. He's still in shock at being accused of liking her.
Speaking of Goo, in "Make Believe", I just love the way he's siting on the couch when he tells her "No Goo no! Remember what happened last time?" He just looks like a little kid, the way he's sitting, and yet he's telling Goo what to do.
I also like when he imagines himself as Rex Carsalot and does those SFX of his car racing.
Oh, and in "Scribbles" when he's running through the halls yelling "Freedom! Let them go! Let them go!" Lol, he's such a good little activist!
Also when he's jumping on Madame F.'s bed yelling "I've never felt so alive! Wheee!"
Wow, and all that was just off the top of my head just now! I love Mac, he has many adorible moments.