My personal, anecdotal experience is that the best charities to donate to are the ones that clearly communicate what their goals are, can demonstrate the good their work does, and are transparent about how they spend their money. I find it tends to be smaller charities that are more effective at this, because their narrower focus means they can explain what they do more succinctly. A charity that says "we rescue all the orphaned seal pups in this geographic area, and it costs £X to rescue, rehabilitate and release one seal" is much clearer about where your money goes than "we help alleviate poverty" without any explanation of who's being helped when, where, or how (if at all. Far too many charities just spend donations on advertising for donations.)

Don't get caught up in promises of vague, non-specific goals. While it might seem like "ending poverty" or "world peace" are the best things to donate to, these are often rather abstract goals that may not even be achievable. The "small stuff" is less grand, less eye-catching, but it's actually these small acts of good, often on a local level, that will actually make the world a better place. Do consider your local charities, the ones who are working directly in your community: that might be your local food bank, hospice care for the dying, wildlife rescue, habitat restoration, keeping the library open, etc. Look for the charities where you can clearly say "yes, the place I live in will be better, and the people in my community will be happier, if this charity succeeds in their goal."

A lot of people will say "don't donate to animals/environment when there are so many humans that need help", and I would urge you not to listen to them. A charity that spends all of your donation on helping hedgehogs or cleaning up litter is doing more good than a charity that says it's helping children in poverty but is actually spending almost everything on advertising. A good charity will be able to demonstrate the positive impact it's having on the world, regardless of whether it's helping children, adults, animals, or the environment.