The best online business for a startup can vary widely depending on your interests, skills, budget, and market trends. Here are some online business ideas that have been popular and have a track record of success:

1. E-commerce Store: Selling products online through platforms like Shopify, WooCommerce, or Etsy can be a lucrative business. You can either sell your own products or source products from suppliers and sell them online.

2. Dropshipping: This business model involves selling products without holding inventory. You only purchase the product from the supplier when you make a sale. Platforms like Shopify make it easy to start a dropshipping business.

3. Subscription Box Service: Curate and sell subscription boxes filled with niche products or items related to a specific theme or interest. Subscription businesses can provide recurring revenue.

4. Digital Products: Create and sell digital products like ebooks, online courses, stock photos, or downloadable templates. You can market these products on your website or through platforms like Udemy or Amazon Kindle Direct Publishing.

5. Affiliate Marketing: Promote products or services from other companies and earn a commission for each sale or lead generated through your referral. Blogging and content marketing can be effective for affiliate marketing.

6. Consulting or Coaching: If you have expertise in a specific area, such as business, fitness, or life coaching, you can offer consulting or coaching services online through video calls or webinars.

7. Content Creation: Start a blog, YouTube channel, or podcast and monetize it through ads, sponsorships, affiliate marketing, or by selling your products and services.

8. App or Software Development: If you have programming skills, you can create mobile apps or software solutions to address specific problems or needs in the market.

9. Freelancing: Offer your skills as a freelancer in areas like writing, graphic design, web development, social media management, or digital marketing. Freelancing platforms like Upwork and Fiverr can help you find clients.

10. Print on Demand: Create custom designs for products like T-shirts, mugs, or phone cases and use print-on-demand services to fulfill orders. You don't need to hold inventory or handle shipping.

11. Online Marketplace: Create a niche online marketplace where buyers and sellers can connect. Examples include Etsy (for handmade goods) and Airbnb (for short-term rentals).

12. Digital Marketing Agency: If you have expertise in online marketing, you can start a digital marketing agency offering services such as SEO, social media marketing, pay-per-click advertising, and content marketing to businesses.

When choosing the best online business for your startup, it's important to conduct market research, identify your target audience, and assess the competition. Additionally, consider your own skills and resources, as well as the long-term viability and scalability of the business idea. Building a strong online presence and marketing strategy is crucial for success in the online business world.