I consider tanky gnar the best gnar. The reason for this is that in each of his forms he scales differently. While in mini form, gnar's main source of damage is from hyper, which scales with AP and deals magic damage, but in his mega form, he is 100% physical damage. In his mega form, attack speed is much less useful since mega gnar is a slow AD caster. In his mini form, attack speed is fantastic and allows you to proc hyper much more quickly and have more control over how quickly you can transform into mega gnar. This makes building armor penetration items and attack speed items a little less beneficial compared to tanky items because if you chose one or the other, depending on your form, you may be more or less powerful. This build allows you to be powerful regardless of your current state; you can chase more effectively as mini gnar without being squished, and you can fearlessly initiate as mega gnar, especially with the mega form's steroid. And do not be scared about not doing enough damage because gnar has great base damage on his hyper (18% of your targets health) and gains AD when he becomes mega gnar.

Enjoy! :D