I found Tripadvisor very helpful for this. Here is a recent thread but there are others. Basically one easy way is to book a remise at an airport kiosk...paying a set price with credit cards and foreign currency accepted. Taxi Ezeiza is one such option. You'll pay at the kiosk and get a receipt at the counter. No need to book in advance - reducing the chances of having to wait around or find the ride you already paid for.
Ultimately paying with cash is best (best price).... and from what I read USD was ok here. CC a second best option.
https://www.tripadvisor.ca/ShowTopic-g312741-i979-k14138145-Booking_a_taxi_booking_com_for_airport_transfer-Buenos_Aires_Capital_Federal_District.html
We can't even buy pesos in advance where I'm from - it's something I will do once in B.A. (That's a whole other discussion).
