For 2,500 EUR you currently get the most USDT on Binance: 2,807.44 USDT. The least favourable exchange (Coinbase) gives you the equivalent of 13.40 EUR (0.54%) less.
Exchanges with a direct USDT/EUR market: Binance, OKX, Bitstamp, Kraken, Crypto.com, Kanga Global, Coinbase. Elsewhere you need a currency conversion or an intermediate currency, which usually adds cost.
Counting only the trading fee and slippage, Bitstamp wins, but once deposit, conversion and withdrawal are included, Binance comes out cheapest.
For 250 EUR the best is Binance, for 25,000 EUR it is Bitstamp. With large amounts order book depth matters more than the headline rate.
Even for 25,000 EUR, splitting the order across exchanges brings no benefit, so one exchange is enough.
EUR deposits: SEPA (Binance, Kraken, Coinbase, Bitstamp, Bybit EU, OKX, Bitvavo, Crypto.com); Card (Binance, Coinbase, Bitstamp, Bybit EU, Bitvavo); P2P (Kanga Global); Revolut account (Revolut X).