The following list of slot machines to avoid is purely based on the mathematical advantage in different types of machines. As for which slot machines are the most fun, it's everyone's personal opinion and has nothing to do with this page.
The more a slot machine costs to manufacture, the more it costs for the casino and the higher the house edge will be (someone has to pay for that extra cost).
The flashier the machine, the more it costs for the casino as well. Every extra animation, sound, bonus game and detail in the actual machine (not just the software) adds to the manufacturing costs.
A common belief in the casino world is that casinos put machines with higher payouts to areas that are visible to everyone - this way there would be a better atmosphere in the casino and players would be encouraged to keep on playing.
Well, numerous sources tell me that this isn't true. There's absolutely no correlation between a slot machine's location inside the casino and its payout percentage. And there's zero proof that such a correlation would have ever existed. So there's little reason to avoid slots that are out in the open.
But a casino's location does matter; for example, Las Vegas casinos on the Strip have a higher house edge than casinos off-Strip, and off-Strip casinos higher than downtown casinos.
Since X percentage of each pot goes to the jackpot - and then the casino has to get its cut of the bet - the slot machine pays back less than it would if no money went to the jackpot.
At some point, when the progressive jackpot is incredibly huge, it makes sense to play them since the amount of money in the jackpot is big enough relative to how much out of each of your bet goes to the jackpot and your chances of winning it.
But it's impossible to tell when that time is since casinos won't publish individual machine's payback information.
"Payback up to 96%..."
Unless the payback of a slot machine is stated specificially (e.g. "Payback is 96%") there's no reason to trust the machine; or, at least, no way of knowing the real payback other than that it's likely less than 96% (otherwise the "up to" makes no sense).
Lower and Higher Denomination
The higher the denomination, the higher the payback; however, the lower the denomination, the less you lose simply because your bets are smaller. For better returns, go to $0.50 slots instead of $0.25 slots; to lose less, go to $0.25 slots instead of $0.50 slots.
Airport Slot Machines
As a general rule, slot machines located in airport terminals have the worst payback percentage of the bunch. The reason is obvious: airport slots have a captive audience and no competition. The operators not only want your money as you're departing town or waiting for a flight, but they have to share the winnings with the airport. Airport slots sometimes have a return-to-player (RTP) of 86% to 89% - which is awful.
Video Slot Machines
Aside from being fancier than mechanical slot machines, video reels take more time (with animations and such) and are therefore slower to play. Since the casino makes money slower, the machine must have a higher house advantage.