Haveno and Retroswap are both solid choices, but Haveno is probably the easiest to get started with. Retroswap requires a bit more technical know-how but can offer better privacy since you can control your UTXO selection for the BTC side. If you're new to this, Haveno is the place to start. Just be aware of the trade-offs on the XMR side too (liquidity, price slippage). Good luck!