In the U.S., at a minimum, you'll likely need to file:
Form 8949 (Sales and Other Dispositions of Capital Assets) with a complete list of every cryptocurrency disposal you have had (e.g. sell, trade, send to a third party, etc.)
Schedule D (Capital Gains and Losses) with an aggregate sum of your capital gains across all asset classes
Form 1040 (Individual Income Tax Return) including your Schedule D information
Schedule 1 (new in 2019): first question about cryptocurrency plus any income (from forks, airdrops, mining, or payments) will go on the "other income" line 21 (for 2019), line 8 (for 2020)
For U.S. users, CoinTracker provides Form 8949, Schedule D, and Schedule 1 which gets you everything you need to complete your cryptocurrency portion of your tax filing. In addition a summary of your overall capital gains for cryptocurrency is provided in the "Capital Gains" section of the tax page
FinCEN clarified in November 2019 that foreign tax reporting via FinCEN Form 114 (FBAR) is not required (including retroactively). The IRS has not released clear guidance on whether or not the Statement of Specific Foreign Financial Assets (FATCA) is required or not, so if you think it may apply to you, the conservative approach is to simply file (more information about FBAR and FATCA for cryptocurrency).
Additionally, if any of your cryptocurrency assets were lost or stolen, you will want to complete Form 4684 (Casualties and Theft). Note: this is only allowed in 2017 and earlier (the TCJA tax bill does not allow for claiming losses on non-physical property starting January 1, 2018).
If you underpaid your quarterly taxes for capital gains, then you will want to complete a Form 2210 (Underpayment of Estimated Tax by Individuals, Estates, and Trusts).
Disclaimer: CoinTracker is provided for informational purposes only. This service is not intended to substitute for tax, audit, accounting, investment, financial, nor legal advice. For financial, tax, or legal advice please consult your own professional. The information on CoinTracker is subject to change without notice. All information is provided "as is." CoinTracker disclaims any responsibility for the accuracy or adequacy of any positions taken by you in your tax returns. Please see our full disclaimer.