Well, the one that wins, of course. If I knew, I'd sit there day and night until the casino was mine!
But seriously.....don't spend your money on books to beat the slots and subscribe to theories. Slot hits are random. There is a random number generator (RNG) which is a computer chip that comes up with numbers that correspond to the reels on the slot. The second you pull the handle or push the spin button, the machine spins and shows you what the computer picked for you. The spin is just part of the razzle dazzle. If you pull or push a nano second later, you'll have a different result.
There are all sorts of theories about the best place to play. The casinos plot the slot machine placement with the precision of an army general. The best paying slots are usually on the high traffic aisles. Makes sense, you are walking by and see a lot of winners, you just have to stop and play. Well, maybe not you, but I sure get sucked in.
Other good places to play are near restaurants or snack bars, round carousels that are elevated so everyone can see them from any angle and by the old change booths. (There's not many around anymore!) I've also had luck near showrooms where the crowd is either lined up to see the show or exiting from the showroom. I've also had a lot of luck at end machines.
The higher the denomination, the better the payback. Its a fact. But on the opposite end, I've seen wins in the thousands on pennies. Hmmmmmm. I've got a real life friend who hit 50 grand on Zorro which is a penny progressive.
You will also get a higher hit frequency in casinos that are off the strip. I like the Station Casinos, but I really enjoy playing at the local casinos. Sit down and within minutes there will be some local who will give you all the casino tips including what is paying and what sucks.
The best machine to play is the one you enjoy playing. If you enjoy playing it, go for it. Slots are computers and we all know how computers behave.....sometimes they are good, but when they are bad, they are soooo evil!