Slots machines Play For fun
Your question is like asking "What destination is the most fun for a low budget traveler?" It depends on your definition of fun.
In any case, there are three main elements that go into a slot players experience,
1. Time on device - given their bankroll and average bet how long can someone play
2. Number of jackpots they hit while they are playing
3. The size of jackpots
For example, let's say the budget is $5. Depending on how you valued the 3 elements above, you could
Play $5 once on a machine where you could win a life-changing jackpot with a small but non-zero probability and lose the entire $5 with near 100% probability.
You could bet a penny at a time 500 times on a machine that could win a moderate sum of money (a few $1000) with a very low probability and lose the entire $5 with near 100% probability and hit no little jackpots at all
You could bet a penny at a time 500 times on a machine that could win more frequent small jackpots (a few pennies to $10).