What is a hard password to guess but at the same time easy to remember?

I would like to use it for an iPhone.

Use the non-simple passcode and just use one letter such as P as it is closet to the unlock button.

A sentence that makes sense to you.
i love dogs… Then change it slightly to read like a license plate, or a bumper sticker.
i<3muTTs, ME*wuv*PuPs, @THEdog#, etc.
Have some small letters, some caps, and a special character or 2.
Best Wishes

Your shoe size + your bra size + your height.