I've been running some cheap Amazon ones that massively improved range. They appear to be a 1/2 wave. I did check them on a NanoVNA to make sure they're actually tuned for 915 MHz, though. One of my T-Beams came with one tuned for 433, so the VNA is probably a good tool to have around.

When I get a minute I want to build a little 1/4 wave ground plane to compare to these cheap antennas.