In Electrum wallet, mempool is the best option if we need to choose one default option for transaction fee estimation.
The best is to use your own node mempool like Bitcoin Core to set the fee. If you do not use full node, you can check the mempool on a third party site.
https://jochen-hoenicke.de/queue/#BTC%20(default%20mempool),24h,weightYou should not rely on the fee a wallet is estimating for you, check the mempool for best fee to use.