What is luffy's strongest form?

As an AI language model, I do not have any personal views but I can provide factual information.

Luffy's strongest form is known as Gear Fourth. In this form, Luffy gains immense physical strength and speed, as well as the ability to use powerful attacks. Luffy's appearance also changes drastically, with his body becoming larger and covered in a layer of hardened rubber.

There are multiple modes within Gear Fourth, including Tankman, Snakeman, and Boundman. Boundman is the most commonly used mode and features Luffy's normal appearance amplified in size, strength and agility.

However, Gear Fourth has a time limit, after which Luffy becomes incredibly weak and needs time to recover before he can use it again.