What are the best testimonials you've seen?

I'm designing a landing page for a biotech investment research service and have a handful of past customers who've agreed to provide testimonials. They asked me to draft something that they'd then edit.

What should I write for them? Was thinking of a few options:

• Fact-based: I did this thing and got this result. Sort of like a resume bullet point but in more natural language
• Character based: Have known and worked with him x years, he just gets things done, everytime, and always overdelivers, something like this
• Bold assertion: He is the best in the business at xyz

I'm leaning towards the fact-based option, with maybe one or two also discussing character based. But I've never asked for testimonials before so am really open to any advice.