I don't know what the best is, or how we'd go about defining such an animal in practical terms outside of catastrophic failures. I use a number of Galaxy and Quantum ballasts (1K and 600HPS). I chose them, because they ran SS cordsets without adapters (there weren't many options at the time). I also have a pair of el-cheapo 400HPS ballasts that I got for something like $75 each BNIB. Ran them for ~1.5 years on 18/6 with zero issues. I've been running the bigger lights in the flowering room for years, too. Everything works like it's supposed to. So what's best? Beats me.
From my perspective, I only care about the final product. The bud. Any piece of equipment pays for itself over and over on the first run, so the cost is largely irrelevant if it actually produces. If a digital ballast fails, so what? It's less than $300 for a 1K. That's not even the cost of an ounce at retail. Just replace the thing and go about your business. Magnetic ballasts, OTOH, are a different story.
Simon