Depends. If you are looking for a FIAT-Crypto-Gateaway there aren't many (legit) platforms. I would go with localbitcoins or hodlhodl. Another option would be to try to find a reputable trader here on Bitcointalk and do a local Cash-Bitcoin transaction. For nearly instant FIAT-Crypto-Transactions you need a decent CEX (e.g. Kraken, Coinbase or Binance) which accepts instant SEPA FIAT-transactions or credit cards.

For Crypto-to-Crypto exchanges, just go with the ones mentioned above (e.g. DEXs like UniSwap or 1INCH or CEXs like KuCoin or OKX). But keep in mind that even if a CEX doesn't require KYC initially, they may require it for larger transactions. That's why you should use DEXs wherever possible.