I've only used
movistar which I had luck with or should I say, good service. I moved from BsAs to another city and was able to get a new SIM at one of their main phone offices. Not a sales store, but the office where people go to pay their bills. They were friendly and helpful. Service in english and they didn't charge me for changing chips (on a phone I bought from them).
There are several other big cellphone companies to choose from since cellphones are almost the lifeblood of Argentine (social) life. I would look into something that would let you roam around the country since, let's face it, you'll want to roam at some point.
It's also fairly cheap and easy to buy a "pay as you go" phone as well. Something small that can be taken to a club and easily replaced if it goes missing (as they tend to do). Mine cost about 130 pesos and then you buy time credits.
An important point about phone credits is to add credits every month, even if you haven't run out. With my phone, if you didn't add any credits within 30 days from the last credit, all previous credits on your phone start to disappear.
The easiest way to buy phone credits is through internet shops. They have a computer connection with the phone company and will credit your account to almost any amount. Next best are the phone cards which you can buy anywhere. You scratch the back like a lottery ticket and it has a code, which you punch into your phone after calling a number on the card.