Here are some expressions you might put in a letter of reference if you want a positive surface-level reading but a... more nuanced literal meaning.

• Nobody would be better in your research group than him.
• His research results are fabulous and incredible.
• His proposals have been described as fantastic.
• You would be lucky to get him to work for you.
• I recommend him without qualification.
• His work fills a much-needed gap in the literature.
• I cannot say enough good things about _____’s commitment, intelligence, hard work, and creativity
• You will be surprised to discover how much X knows about the subject.