As jewels go, this is a standard appearance for the Queen. Her standard day set of Queen Mary's Button Earrings and her Three Strand Pearl Necklace are worn today, with appearances early on from other options like the simple Pearl Stud Earrings. A brooch, of course, is required; though floral motifs are obvious choices, they share space with other more general designs and no one favorite stands out from the first 60 years of the Queen's reign.
The real standouts at the Chelsea Flower Show are actually not the jewels, but the headgear, to tell you the truth. Up until 1967 the Queen wore a hat, but from 1968 on she's gone hatless, arriving in either a headscarf or one of her hairnets studded with bows or little flowers...which is an interesting look, to say the least.
A selection of flower show appearances:

1955: Australian Wattle
1958: Queen Victoria's Bow
1959: Cullinan VI and VIII
1960: Australian Wattle
1961: Queen Victoria's Bow
1964: True Lover's Knot
1966: Modern Ruby
1967: Teck Corsage
1968: Queen Adelaide's Brooch
1975: Australian Wattle
1977: Cullinan V
1981: Prince Albert's Sapphire
1982: Cullinan V
1984: Modern Ruby
1985: Modern Ruby
1993: Australian Wattle
1994: Australian Wattle
1996: Queen Victoria's Bow
1998: Pearl Trefoil
1999: Cambridge Pearl Pendant
2000: Cambridge Pearl Pendant
2001: Jardine Star
2002: Australian Wattle
2003: Frosted Sunflower
2004: Queen Victoria's Bow
2006: Jardine Star
2007: Cullinan V
2008: Kent Amethyst
2009: Australian Hibiscus
2010: Queen Mother's Palm Leaf
2012: Jardine Star
Photos: British Pathe/PA/BBC/Telegraph/Getty Images/Reuters
During her 2012 visit, the RHS presented their Patron with a special gift to mark her Diamond Jubilee: the Chelsea Iris Brooch. From 2013 to 2017, she wore that brooch for the Flower Show. In 2018, she opted for the Small Pink and Diamond Brooch; 2019's choice was the Flame Lily.