Everyone has their own idea of "best", and trying out a bunch of slots in freeplay is a good way to see what you like.
Some slots I have a love/hate relationship with. Break da Bank (the three-reel version) is one of these. Minimum bet is $5, but I can have a hard time tearing myself away from it.
I really enjoy the Wheel of Wealth 3 liners. But at $2 00 - $3 it can sure eat it fast. They have provided some big wins for me.
Cashonova remains one of my favourite slots. The Henhouse bonus doesn't pay huge wins (around 30x mostly), but they come along pretty frequently, and there's another Egg Feature that can pay out quite largely, but sometimes 10x or less. I like pick features personally, gives me an illusion of control, lol.
Kung Fu Monkey is another one I enjoy.
Still like Mega Moolah (the original) because of sounds and graphics and music.
I enjoy Gypsy Queen, but it's awful on autospin. Again, I like the music.
Osbournes is fun too... 3 different bonus features.
If you look through the Winner's Screenshots threads you will usually see higher variance slots giving really big payouts. I'm not a huge fan of most of them, but it is all some players play. When I do play, I don't usually play for long.
The "fruit machines" are very interactive to play, but read as much as you can here about them, and tackle only one or two at a time.
If you mean "best" as in the highest percentage payouts, I think most of them are around the same, it's just how they are spread out. Except the progressives, which part of the bet goes to the progressive pool. So if you don't win the progressives, you won't get that part of your bet back. The fruit machines might pay back lower as well.
Sounds, graphics and how fun the bonus round is are big factors in my play.