I’m sure you can still find some of my posts archived at the old Washington Fly Fishing forum. All of my experience has been in Argentina and all DIY; I have never stayed at a lodge or fished with a guide (although I have made some Argentine friends who have taken me out a few times). This gets harder every year, as the huge private estancias (ranches) increasingly are posting their lands and only providing access to expensive lodges who pay them for the privilege.
I will just make a few observations without giving away too much in a public forum.
In the far south (not so far as Tierra del Fuego) my preference is for the spring creeks over the big rivers. The rivers have sea run browns in season, but with them, crowds. I’ve never been crowded on any of the spring creeks; heck, I’ve never seen another fisherman! Look for them on the Chilean side of the border north of Punta Arena and in Argentina south of El Calafate. They may be spring creeks, but 3X tippet is your friend. Enough said. Be sure to take time to visit Torres del Paine, the glaciers near El Calafate, and Fitzroy near El Chalten. If El Calafate is your first stop in Argentina, you can get a license at the national park Headquarters in town. The license is good in all five southern provinces of Argentina.
I would love to drive the long lonely stretch of hwy 40 from El Calafate to Esquel sometime, and I would certainly bring my fly rod, but from what I understand there is little public access to good water through there. Enjoy the drive; let me know what I have missed.
From Esquel north to Lago Alumine there is so much fantastic water you can’t go wrong, which isn’t the same as saying it’s all equally good or readily accessible. My experience is not so extensive that I can say with confidence what will fish well and what won’t. I will mention a few of my favorite rivers in this part of Patagonia, knowing that I am not giving any secrets away. From south to north, consider fishing Rio Rivadavia, Rio Chubut (below El Maiten), Rio Chimehuin and Rio Malleo (I like the lower end on the Mapuche reservation), and Rio Alumine. I like Junin de los Andes for a base in that area and always stay at the classic Hosteria Chimehuin. These recommendations are like telling someone who is going to Montana to fish the Bitterroot, Madison, and Gallatin, without naming any of the lesser known gems. You can’t go wrong, but there’s more to find if you have the time to explore.
Have fun, and report back when you return!