I am just wondering which is the best tax filing for my small business? It is an LLC and I did not file anything within the first 75 days (I didn't realize I only had 75 days)

The good news is that missing that 75 day window almost certainly is not a problem, because your LLC does not need to file anything special to be taxed at all. By default, the IRS treats a single member LLC as a sole proprietorship (reported on Schedule C with your personal return) and a multi member LLC as a partnership (Form 1065). Neither of those requires an election or a deadline. So if you have not filed any special paperwork, your business is likely just being taxed the default way, which is completely normal and valid.

The 75 day rule you are thinking of applies only if you wanted your LLC taxed as an S corporation instead, which some business owners do once profits get high enough to make the self employment tax savings worthwhile. That election (Form 2553) normally needs to be filed within two and a half months of either forming the LLC or the start of the tax year. If you missed it but genuinely intended to elect S corp status and have a reasonable explanation for the delay, the IRS often allows a late election under its late relief procedures, sometimes up to three years and 75 days after the original intended effective date.

So the real question is whether you actually wanted S corp treatment or just default treatment. If your net profit is under roughly $100,000 to $110,000 dollars a year, staying with default treatment is usually simpler and fine. If you are earning more and self employment tax is getting painful, it might be worth exploring the late S corp election.

If you want, a Keeper tax pro can look at your numbers and tell you whether S corp treatment would actually save you money before you go through the late election process.