You can add Youtube videos to Slides quickly and easily.
Doing this means there are NO ADs, NO COMMENTs and NO SUGGESTED VIDEOs on the sidebar.
Click INSERT → VIDEO and then you can search Youtube or paste in the link of the video that you want to add.
You can also add videos you have made from your Drive.
Another cool thing you can do with video in Google slides is you can decide on the start and finish time if you don't want to show the whole thing.
Once you have added the video, click on it and then you will have a FORMAT menu on the right hand side.
Here you can change the start and end time of the video and you can also change it from playing when it is clicked to having it play automatically as soon as you go to that slide.
Check out the screencast below to see how this works.