As far as I'm aware, there's no hard absolute data for this (at least, not for gen 1 and gen 3). That said, there have been some people trying to figure things out and the following is generally accepted as "true" by many players:
All the slot machines have their odds randomly generated each play. As such, there is no single slot machine that is good for every player all the time.
Generation 3 includes two sets of games: ruby/sapphire/emerald and fire red/leaf green.
For Ruby/Sapphire/Emerald, the slots are again fairly random. You may be able to play a slot a couple of times, and if it pays out both times, it might be "hot" and worth continuing on.
For Fire Red/Leaf Green there has been speculation that the machine on the far left, directly below the woman in that aisle has slightly better odds. There isn't any absolute proof of this fact though and may simply be confirmation bias (you think it's better odds, so it feels like it is because you notice your wins more). FR/LG also have an interesting aspect to its algorithm: when the slots start spinning, the game has already decided the % chance of a win/loss. That said, it's not until the last slot stops that the game chooses what the payout will be. This means that if you have an emulator with save-states, if you save after starting to spin but before stopping any of the slots, and then repeatedly load that save, you will likely see either consecutive wins, though of varying amounts, or consecutive losses.