D locks that are difficult to cut with an angle grinder are well worth the cost if you value your bike but they are expensive.
Litelok & Hiplok D locks have been proved the best.
A simple bike cover is also a great deterrent (thieves won't waste their time if they can't readily see what they're stealing).
My favourite security device is to park mine next to something easier to steal.
Whatever lock you chose is best if it locks/chains your bike to solid immoveable infrastructure

2 recommended D locks are here:

<a href="https://www.litelok.com/products/litelok-x3-moto">https://www.litelok.com/products/litelok-x3-moto</a>

<a href="https://hiplok.com/product/hiplok-dx1000/?motorcycle=true">https://hiplok.com/product/hiplok-dx1000/?motorcycle=true</a>