for me, depends on the cartridge
40 and up boomers - Brno 602 or CZ 550
long belted mags weathebys, STW etc - classic M70
short belted mags - M70 or M98
standard (30-06) - pre-64 M70, M98, ZKK/CZ, HVA 1600
As far as which one is safest under a catastrophic failure (case head failure i assume), me, I'd want to be behind a good military M98 with a full C-ring and thumb slot
Between the 1640 and M70, the 1640 has better gas protection with the full sized M98 style shroud that completely blocks the left rail opening. The 1640 also has a true third safety lug, the M70 uses the bolt handle as a safety lug, which would be great, except it is soldered on. I don't personally worry too much about gas handling or third lugs, though others may. The M70 has a much better trigger and safety, as well as shorter firing pin travel (faster lock time). It also can take a 3.600" cartridge with minimal fuss. Either, if used in the original cartridge configured for, will be reliable and amply strong.