Whatever is cheapest that'll run well in my guns.
I started shooting not too much before
The Craze of 2012-14, so I've really sampled quite a bit of what's available for 9x19, which is what I mostly shoot - and truthfully, in terms of range fodder, I've honestly never seen that much of a difference with the exception of unique-gun-to-unique-ammo tolerance-stacking issues. For example, my buddy's Roland Special build likes to toss 124 gr. Blazer Brass: his groups using this specific ammo is about 2x as big as on virtually *all* other ammo. Similarly, with my main training/range-use 4.5-inch XDm9, I can't seem to get reliable function from 115 gr. Magtech, while my EDC XDm9 3.8 Compact locks up to the point that it'll require mortaring when I try malfunction medleys with it using once-fired AE brass.
I've shot tens of thousands of rounds of both 115 gr. WWB and 115 gr. Blazer Brass, and both have treated me well. I think that each of their known FTFire rates in my guns have been on the order of 0.5% (zero-point-five percent) or less - as a concrete translation, that would be less than 50 problematic rounds out of each 10,000 rounds fired.
I've also pushed a fair bit of Remington UMC, and aside from having some spectacular muzzle flash in low-light, I didn't find it to be any dirtier or otherwise less desirable than any other range-fodder:
^ You can see the target lit up by my WML (Surefire XC-1), but that muzzle blast and tracer-effect? That's all UMC!
I've shot a bunch of Freedom Munitions - both new-manufacture and their remans during the worst of
The Craze. I'm actually just now about 1,500 rounds out from finishing my last order from them, a case of their new manufactures using X-Treme cases, which dates back to 2016. I had no problems hitting a Challenge Targets ABC-zone steel, at the 45 yard line, even shooting with just one hand, from a gun that's seen now close to 55,000 live-fired rounds and was last cleaned in the spring of 2019.
Fiocchi and S&B's 115 gr. fare well in my XDms, too, as do the various weights/lengths of Speer's Lawman series. Lately, I've really learned to love Magtech, but as I noted above, the 115 gr. variety really doesn't play well with my main training gun for some strange reason.
Honestly, though, if there's some brass case that's hovering around 18 cents per, you can be pretty sure I'm hitting that "buy" button.