When Ben first transformed into Wildmutt and teased Gwen.
How about when Ben first turned XLR8, ran all the way back home, and hung Cash and JT by their undies....the same humiliation they inflicted upon him at the start?
When the Tennysons are visiting Yellowstone Park, all Ben is doing is fiddling with the Omnitrix, trying to turn into XLR 8 so he zip back towards a burger place for lunch. Gwen calls Ben out on his superficiality, and Max assures the kids that they can "Live off the land." After seeing Max uncover a nest of worms, Gwen quickly starts helping Ben with the Omnitrix.
In "Ben 10,000", Ben and Gwen play Rock, Paper, Scissors. They're interrupted by a time travel portal. The caption said "Ben would have won this time".