I haven't play the Asphalt 8 for a while so I can't be sure when did it happen.
But after Fifteenth Anniversary Update, I think there has no such thing as credit farming--Multiplayer credit wasn't deducted like any others. Almost all races, player can get only 300 to 500-ish credits so base credit is much less than that.
Just before Fifteenth Anniversary Update, only daily events--TLEs give 3,000-ish credits and Multiplayer credit was quite same like last time. 4+ player joined with 2 laps and if player finished first, player can get 6,000-ish credits.
Now almost all the races will give 300 to 500-ish credits only, include bike race. Current situation, one possibility of farming credits will be Time Attack on bike race. For example, Season 10 Ultimate Showdown - Patagonia Sprint 1 Time Attack, can finish 35 to 40 seconds--Suzuki Hayabusa R1686 (10-10-6-10). And can get 520+ ish credits.
So Time Attack with track requires lots of stunt will be best choice for now.
Or Multiplayer is probably the quite good way to farm credits--but in Windows, waiting time is too long and someimtes you can't win the race because not the best MP car you have. Not sure about other platform.
R&D will give slight higher base credits--like 600. But player must spent extremely high amount of credits for upgrade.