Probably depends where you live. In Europe bank transfers (generally free or very low cost and instant/within 24 hours).

If we're talking about alternatives for giving money to a charity, I strongly recommend opening up the contact page of your preferred charity, calling them and just asking them what method they would prefer for you to send them money. Charities often have preferences that allow them to save money on transaction fees. For example they might have a discounted rate with a particular credit card company or prefer a physical check, or want you to pass it through a fiscal sponsor. Especially for small charities -- donors are the lifeblood of charities, so they will bend over backwards to help you give!

Source: I run a charity.

The good'ol cash or physical check will always work. Don't believe so called the 'new money': you lose all control of your own money and at someone else's mercy.

Bitcoin is great. For people who don't use Bitcoin I use Square Cash or Venmo (owned by PayPal).