In EU I think it has to be relai since you can get it down to a 1% fee.
Pocketbitcoin has 1.5% fee.
Then you have the p2p exchanges like bisq, peachbitcoin, robosats, hodlhodl in which the very nature of the exchange implies that you buy at (that specific) market price, with the downside being that those markets are very ilquid and thus have a premium above KYC market price.
Relai seems nice but it does not accept free orders, you have to create manually and order every time an submit a SEPA trasfer with the exact amount. Exception for the recurring orders, where however you cannot change the value after you start one.
This resource used to be fairly comprehensive (and accurate). But it has been a couple years since I've referenced it and have no idea if nowadays it is still either of those.
In the EU zone Bitstamp.net has zero fee under €1000/month. Minimum buy €10. Your can open limit orders below the market price and try to save some %. It has an API so you can easily script a DCA routine. But it is full KYC
Pocketapp.com has 1.5% few, "light" KYC under €900/month and it is easy as send an (instant too) SEPA to automatically buy at market price with direct transfer on your own wallet.
Fees are related to convenience, security and privacy, make your choose :)
If you are looking to Buy bitcoin with fiat, then the methods available will differ based on a few factors:
Where are you located (country)?
How much are you looking to buy?
What payment methods do you have available?
How soon do you need access to the proceeds?
Is financial privacy important?
With a P2P exchange, you can create a Buy offer and set your own price (e.g., 4% below spot). You are competing with other Buyers, so if you want to find a Seller you'll have to offer a competitive price. But most trades there that happen from a Seller Taking a BUY offer result in the Buyer (i.e., you) paying below spot. That's sometimes referred to as "negative fees'.
If you are needing your coins sooner, you can always Take a Seller's trade offer, but in that instance you are the one paying the premium, and the Seller benefits from "negative fees".
tl;dr: For the best price, be an offer maker, not an offer taker.
pocketapp.combut pocketbitcoin.com