Cheeriest is the superlative form of cheery—the most bright, lively, or good-humored among a group.
Even the cheeriest optimist couldn't find a silver lining in that news.
She was the cheeriest person at the funeral, which struck some as inappropriate.
1. festive — brightest, Superlative of luminosity
2. joyful — happiest, Superlative of joyful; merriest (Most full of joy); sunniest (Most filled with light)
3. bright — sunniest, Most filled with light; warmest (Superlative of hot-adjacent)
4. upbeat — peppiest, Most full of vigor; perkiest (Most cheerfully energetic); jauntiest (Most cheerfully self-confident); zippiest (Most lively and energetic); bounciest