Short answer
Start with the brand, then say what the product is, then add the details a shopper would filter by, such as model, colour, size and material. Keep promotions out. Titles can be up to 150 characters, but many places show only the first part. The free product title builder on this site does it for you.
Free tools for this
Why the title matters so much
Google matches shoppers' searches mainly against your product title. A title that says what the product is, in the words people type, can show for many more searches than a clever or vague one.
The order
Brand, then what the product is, then the details that tell it apart: model, colour, size, material, pack size. Put the most important words first, because many places show only the start of the title.
A shopper searching for a black Nikon camera body should be able to see all three of those words in the first part of your title.
Try it now: Product Title Builder → · Merchant Center Readiness Check →
Length
The limit is 150 characters. You do not need all of them. A clear title of 60 to 100 characters is often better than a long one stuffed with every keyword.
What to keep out
Promotions such as sale, free shipping, best price or limited offer. Google Shopping does not allow promotional text in a product title, and it can get a product disapproved.
Capitals for emphasis, symbols, repeated words and your shop name. The brand belongs in the title, and the shop name does not.
Different from your page title
Your store's page title is written for the customer already on the page. The feed title is written to match what shoppers search. They can differ, and they are allowed to.
Doing it across a catalogue
One title is easy. Hundreds need a rule, applied the same way every time, then checked for products that break it. This is the part that tends to go wrong, with brands missing on some products and attributes spelled four ways across others.