The best level to mine for diamonds in Minecraft as of version 1.21 is around Y-level -53. Diamonds spawn below layer 16 but become increasingly common as you dig down toward bedrock. The optimal level is just above the lava lakes that form at Y-level -54, making -53 the safest and most productive level to mine diamonds without the risk of falling into lava

. More specifically:

• Diamond ore and deepslate diamond ore are most abundant between Y-levels -53 and -59, with the highest concentration near -59.
• Mining at Y-level -53 keeps you just above the lava lakes at -54, reducing danger while maximizing diamond yield.
• This applies to both Minecraft Java and Bedrock editions

In summary, mine at Y-level -53 to maximize diamond finds while staying safe from lava hazards. Exploring caves near this level can also be effective, but strip mining at -53 is a common and efficient method