You'll probably get various responses, because there are numerous farming strats, including in the same league mechanic.
For basic precursor tablet, the most expensive mod is „waystone drop chance” -- something like 145% will instantly sell for a couple of divs even with other crappy mods on it.
Other generally good mods are „increased rarity” (that is if you farm for rarity, because it's super-profitable to go 0 rarity, or even negative rarity -- as I previously said, there are multiple strats), „increased number of rare monsters” and „monster effectiveness” (which is the main way to scale quantity and on top of that it increases experience gained). Speaking of, the good rolled experience tablets are also pretty valuable. Additional modifier is also good, additional mod for uniques is the best strat for boss-farming and rogue exiles.
Anyway, what I'm trying to say is you should go to poe2db and search the full list of mods and you'll automatically realize there are tons of strategies and ways mods synergize with each other in order to create farming strategies.
Good luck!