Ask HN: Best Pre or Post Processor Advice

I was just wondering what people’s (educated) opinions were on which pre or post processor are the most powerful currently and looking forward.

I’ve been working with Sass for a fair time now, but am advising a client who currently uses Vanilla CSS on which processor will be the most beneficial to move to.

I love using Sass, but wondered has LESS caught up? Is PostCSS worth looking into?

Any thoughts would be much appreciated!