I think the FR/LG slot machines are relatively easy to play. I remember my brother and I taking turns with those machines during a drive to Vegas when we were kids. We hit so many jackpots! Despite being Rocket-run, those slots don't seem as rigged as some of the other ones.
The slots in D/P/Pt are kind of friendly too, as long as you keep at it. I still remember getting 10 consecutive bonuses and winning that Explosion TM the first time. ^-^
For the games that give you a choice of slots and another game (Card Flip, Roulette), I preferred the other game. Slots were evil in those. Also, I love Voltorb Flip.