In principle, all the larger exchanges can be recommended, but it doesn't hurt to do some research upfront, because the fees are different. If you don't care about trading but want to get some and hold for a longer time, I would choose an exchange with low fees. There are several steps, you have to make a transfer of fiat money to the exchange first, and I'd have a look what the cheapest options are there. I use € for example and most exchanges offer SEPA transfer of fiat with zero fees, and signing up to be able to use SEPA definitely makes sense. Most important is though whether an exchange allows the transfer of ADA to a private wallet, and some exchanges don't offer that yet.
Then, on some exchanges there might be a difference in the fee depending on the order type you use. There is a maker fee for limit orders that is usually lower, and a taker fee for market orders that is usually higher. A (post) limit order is usually cheaper than a market order for that reason, but the differences are small: On Kraken, the maker fees start at 0.16% and the taker fees start at 0.26% for example (source).
Then, there is usually also a fee for withdrawing from an exchange, in the case of Kraken the fee for withdrawing ADA and transferring it to your wallet is 0.6 ADA.
I use Kraken because I like their trading platform, but Binance works well too and is cheaper, but I heard of issues with not being able to withdraw ADA for a few days after purchase or around the epoch boundaries, that might not be an issue for you though. I also once tried buying ADA via Bitpanda and they have comparatively high fees, and it turned out that I couldn't transfer ADA to my wallet (but as I have been told, this should be possible now), so I had to exchange it to a different coin, transfer that to Kraken, exchange it for ADA and then transfer the ADA to my Yoroi wallet. As you can imagine, the fees accumulate in such a case, so please be smarter...