$USDC and USDT are the two most popular stablecoins, pegged to the USD at a 1:1 ratio. USDC (USD Coin) issued by Circle is more transparent with regular audit reports, and its collateral assets primarily consist of cash and US government bonds. USDT (Tether) issued by Tether Limited has a larger trading volume but has faced controversies regarding transparency and asset reserves. Both are widely used in cryptocurrency trading, but USDC is often rated higher for its reliability and transparency.