They say it doesn't matter how many elements a system has -- it's only as strong as the weakest one. That's probably why no milanesa is a great milanesa unless it starts with noble techniques and materials -- not expensive, but effective. Here I share the workflow I use. I've never worked as a cook, but I've been eating milanesas (of all kinds) on a weekly basis for 35 years, and I've been making them for two decades.
What meat to choose for milanesas
From the traditional butcher's selection, it's perfectly fine that top round (nalga) is the definitive cut for milanesas. It has great flavor, absorbs whatever seasoning you add, it's tender, even, yields well, and is memorable -- because you can get those massive milanesas that take up the entire plate. And on top of that, they come out shaped like Argentina. Although it's one of the most expensive cuts for milanesas, it has rightfully earned its place as the homemade standard and the go-to for serving on a plate or between bread -- by the way, have you read Juan's recipe for homemade bread yet?--. From one kilo of top round, you get 8-10 fillets.

Another classic is the eye of round (bola de lomo), though it varies more depending on the piece you get. It's flavorful but doesn't quite convince me because the center tends to concentrate fat and stretch a lot. Because they're uneven, they yield less and are very unpredictable for sandwiches. From one kilo you get 7 to 9 thin steaks, just behind top round, and it's usually a bit cheaper.

The bottom round (cuadrada), on the other hand, is even, yields well at a good price, and is flavorful. But it can also be tougher. From one kilo you can get between 10 and 14, depending on the part of the piece they cut for you, and that also determines whether they come out thin and long or nicely squared. It's usually priced at the same level as eye of round or slightly below.

All three cuts are flavorful, reasonably priced, and yield well. That's why the choice comes down to personal taste, trust in your butcher, and intended use. Top round works for everything, eye of round is better for plating (because of its size), and bottom round is better for sandwiches (because of its shape).
Disclaimer 1: about peceto (eye of round) and cuadril (rump steak)
Peceto milanesas are expensive and kind of a trap. Well seasoned, peceto is flavorful, but for 1 kilo of meat you have to bread 200 thin slices. Either that, or they have to cut them thick, and you get less yield. And they're kind of a trap because to make memorable homemade peceto milanesas -- fried or oven-baked with olive oil to unlock all the flavor -- you need your own willpower and especially other people's. You can work a lot and end up with tiny, bland, overpriced milanesas. As for cuadril, I don't like its flavor in milanesas, even though I love it as a grilled steak. If you like it as a milanesa, that's perfectly fine.
Step-by-step to prepare milanesas
One meter of counter space and three key elements: meat, egg, breadcrumbs. Three stations to which we'll add only what's essential.
The meat: first I separate the thin steaks on a cutting board, trim off fat, membranes, or tough bits of skin; sprinkle fine salt on both sides + pepper only on top; and let them sweat a bit.
The egg: per kilo of meat, I beat four eggs with a teaspoon of fine salt and a few good gratings of nutmeg (or a level tablespoon, if using ground); all well beaten until the liquid has an even consistency, which allows for clean and effective breading.
The breadcrumbs: for the dry element I use a mix of breadcrumbs and/or breading mix + dried parsley-garlic seasoning (provenzal), because chopping fresh garlic and parsley is a time sink that breaks the convenience pact between humans and milanesas.

Then, I put an exaggerated amount of breadcrumbs in a wide dish or on newspaper, if I have some; dip the thin steaks in egg, toss them onto the breadcrumbs, cover them, and press down with an open hand. The right pressure for breading is the push, not the punch. Some people punish the meat and their knuckles out of pleasure, anxiety, habit, or mimicry, and end up with see-through milanesas. There's no need. Open hand and pressure. Flip and repeat.
The ideal milanesa should be several millimeters thick, with the meat taking up most of the height, yet not so thick that it complicates a quick cook -- which is the other thing we need from them.
The time this preparation takes depends on several factors, mainly the level of weed smoked. I take about an hour, but in 30 minutes or less you can prepare over a kilo of high-quality homemade milanesas, ready for the week.
In summary: meat with salt and pepper, egg with salt and nutmeg, breadcrumbs with breading mix and dried provenzal. With regular or olive oil, oven-baked on a sheet or pan, or fried in a skillet, to the desired doneness. I like my milanesa dry when it goes with salad and pliable for my sandwiches. I like milanesas, period.
Disclaimer 2: chicken ones are milanesas too
To make chicken breast milanesas I usually follow the same method, swapping out a few elements. The already-sliced breast fillets can get a squeeze of lemon. In the egg, mustard instead of nutmeg. And in the breadcrumbs, a bit of oats, panko, or polenta (though I use the same mix as for beef).

The final trick
If you've made it this far, there's a bonus. My favorite meat for milanesas is the tenderloin (solomillo), a fantastic tube of meat that yields thick, tender, and flavorful milanesas. At markets they sell it pre-sliced but with a brutal markup. I prefer buying whole tenderloins (1.5-1.8 kilos) and working them myself: from the clean tenderloin I get 700 to 900g of meat for milanesas, from the meaty part around the tenderloin another half kilo of thin steaks, and from the remaining scraps I get decent meat for a sauce.
Breaking down the tenderloin adds time and effort to the operation, but I make up for it because a single process provides me with meat for different preparations. But above all because the reward in flavor is worth it. Like everything about the milanesa -- a national cultural totem.