(I always thought Cera was larger. Littlefoot was bigger than her in the first movie, but he certainly doesn't look even bigger now, in the later films. Cera looks larger, though I'll have to check to be sure.)
I'd say Tippy's mom. She's likely too large and heavy for Dil to successfully drag into the water, if they were near a lake. And if Dil confronted her head-on, she would need to contend with the tail. Even though Tippy's mom's spikes are small, a blow with her tail alone would cause damage. And Dil isn't exactly swift on her feet.
Chomper vs Guido