I subscribed to SlideBazaar a few months back and I can say firsthand that the templates are superb. Their after sales support is super nice as well.
Microsoft PowerPoint - Microsoft PowerPoint empowers you to create clean slideshow presentations and intricate pitch decks and gives you a powerful presentation maker to tell your story.
SlideModel.com - Create attractive presentations with our professional PowerPoint templates, slide designs, and themes. Use our PPT templates and slide designs to give unique appearance to your presentation.
Google Slides - Create a new presentation and edit it with others at the same time โ from your computer, phone or tablet. Free with a Google account.
Slidesgo - An extensive catalog of free Google Slides themes and PowerPoint templates.
Prezi - Welcome to Prezi, the presentation software that uses motion, zoom, and spatial relationships to bring your ideas to life and make you a great presenter.
SlideTeam - Transform rough sketches, notes into professional PowerPoint presentations.