Let's Talk PnP Systems and Skill Checks!

I'm looking for opinions on the great variety of skill checks that so many different games have. What systems in particular have really stood out to work well? What systems did you absolutely hate?

Do be specific, I'm talking about the actual method of rolling. Various systems include:

the keep system, where you roll X amount of dice and keep Y. a skill check to climb might be climb skill + strength, keep strength. it could work out to something like 4d6 keep 2.

the d20 system. this one is really popular. basically, you roll a d20 and add the bonus from the main stat and skill. a climb check might be d20 + strength bonus + climb skill.

the "success" system. with this system, you might roll strength value + climb value amount of d6s, and every roll of 4 or 5 or 6 counts as a "success". to climb a given wall, you might need 3 or 4 successes.

I know these are just a few of the very many systems, but these are indeed my three favorite. I'm looking to discovery as many as possible, as great things build on those that come before.