What are the rarest 50 Cent Coins?

Could you elaborate on the topic of the rarest 50 Cent Coins? I'm particularly interested in understanding which denominations and editions stand out as the most sought-after among collectors. Are there any specific years, mintmarks, or unique features that make certain 50 Cent Coins exceptionally rare? Additionally, I'd like to know if there's a significant difference in the value of these rare coins compared to their more common counterparts. Lastly, are there any recommended strategies or resources for identifying and acquiring these prized coins? Your insights would be greatly appreciated.