Gringoboy -
Some that I think are impressive, depending on what one is expecting:
Kensho - was open, but I figured they'd kill me if I made them all eat vegan in BA
In retrospect, I think we should have gone there.
Resto in the Sociedad de Arquitectos - was open, food is always amazing in my experience. I thought the decor was maybe a bit simple for our one meal together, but food-wise it would have been ideal.
Adentro - was A++, even for me as a vegetarian, but they are closed right now.
Milion, in the garden or on terrace upstairs - impressive if you are just having a coffee in the afternoon, or a drink and maybe some tapas, in my opinion. Main courses are not impressive and are sometimes shockingly bad.
Croque Madame at the Museo de Arte Decorativo - very impressive if taking someone for just a coffee while out on a walk, or a drink on a beautiful evening. Food not good enough, in my opinion, for what I was going for here.
Floreria Atlantico - Impressive to a visitor for a casual night out and some cocktails.
Arkakao - the only ice cream place in BA that I recommend.
El Ateneo Gran Splendid - for something to drink, never eaten food there. If all I've told someone is 'a café in a bookshop', they are impressed.
Amici Miei, private balconies for 2 - there is nowhere more impressive to take a visitor on a Sunday for lunch on a nice day, in my opinion.
Park Hyatt Palacio Duhau - Never fails to impress for a bottle out on the terrace, and they usually have an affordable bottle of something - they used to have a nice white wine for something like $6 USD.
La Vinería de Gualterio Bolivar - I've been twice and enjoyed both experiences over lots of wine with friends, but have heard it can be hit and miss.
Pura Tierra - had a lovely dinner there with some friends, plenty of wine. Only issue was that they made a mistake with all three of my first courses, and none were vegetarian (!). I caught it in time, luckily, but three times in a row is just careless - it's why I never went back.
Frank's Bar happy hour (in Algodón Mansion hotel): they used to have live jazz, so it was a great way to impress visitors, especially as the glasses of wine - from their own estates - were something like $2 USD/ glass. Was perfect with the simple, cheap food they served and the jazz in opulent surroundings.
Unik - great design, wonderful food.
Alvear Palace Hotel - I know visitors are often impressed by the afternoon tea in L'Orangerie, which includes a glass of champagne. They have a gluten free option and it is usually great value for money, as long as you don't order another glass of bubbly...
La Pecora Nera - Impressive to visitors for a spontaneous lunch while walking around the neighborhood, the lunch special is very well-priced.
Aramburu and i latina - people I know have been impressed and I can't wait to go to both. The group I went to Elena with ate at Aramburu bis and said the food was really, really terrible.
If people are genuinely interested, I'll try to think of more. I appreciate the suggestions and am looking to explore some new places this summer.