According to multiple sources, including the Royal Mint, the rarest 50p coins are:

1. Kew Gardens 2009 - mintage of 210,000
2. Olympic Football 2011 - mintage of 1,125,500
3. Olympic Wrestling 2011 - mintage of 1,129,500
4. Olympic Judo 2011 - mintage of 1,161,500
5. Olympic Triathlon 2011 - mintage of 1,163,500
6. Peter Rabbit 2018 - mintage of 1,400,000
7. Flopsy Bunny 2018 - mintage of 1,400,000
8. Olympic Tennis 2011 - mintage of 1,454,000
9. Olympic Goalball 2011 - mintage of 1,615,500
10. Beatrix Potter (various designs) - mintage of 1,700,000

Its worth noting that rarity isnt the only factor that determines a coins value. The condition of the coin and whether it has any errors in its design can also increase its value. Additionally, the way in which it is sold can also determine the coins value, with some collectors bidding vast amounts of money on eBay or at auction, while others opt for more robust valuations by selling via a coin dealer.