I can't speak for today, I've been off heroin since 2018.
But prior to that, I think the best heroin I ever had was some allegedly uncut heroin we (me and my partner) got from our dealer once. It was expensive, but even using a quarter less of it than we normally would was still significantly better than normal.
Other than that, I've just found it to be hit and miss. Some batches seemed to be better than others with no clear reason.
There was a period where I'd swear the heroin was better simply depending on what time of day we ordered it...
It's weird, I've never been able to work out any consistent reason why it was sometimes better than others.
Since me and my partner at the time always did our shots together, from the same batch at the same time and the same quantity. We could compare experiences and it certainly seemed to be real rather than something psychological. Sometimes I'd think a batch was better than he did, but usually we were both in agreement.