The poll includes only character who appear or are referenced in the films and TV shows (and does not include the Father, Son, and Daughter from The Clone Wars' Mortis arc, due to their nature being somewhat ambiguous). However, the "other" category exists if you feel that there is a character missing from the poll who deserves to be counted in the big leagues.
"powerful" for the purposes of this thread should take into account both innate potential and level of skill/training/experience eventually acquired. Character opinions can be taken into account, but conclusions should be based primarily on observed feats or material evidence such as midichlorian counts.
For my part, going off their performances and attributed capabilities in the films and TV shows, I'd give the title either to Sideous, if one attributes the Galaxy-wide shroud of the Dark Side directly to his powers, or else to Luke, Snoke, or Yoda in The Last Jedi. (possibly Plageius, as well).
Anakin had the potential to be the strongest, perhaps, based on his role as the Chosen One and his midichlorian count, but never fully realized it.
Rey may surpass them all in time, given her rapid acquisition of strength and skill in the Force, but I don't think she yet has the experience or depth of understanding of the Force to be on the very top tier (note her complete inability to counter Snoke's telekinetic attacks in TLJ).
I'd probably tentatively vote for either Luke or Yoda at this time, based on the available evidence of which I am aware (yes, Yoda lost his duel to Sideous- but whatever the RotS novelization says, that always struck me as a very close fight, and Sideous fortunately never learned how to throw lightning bolts from beyond the grave
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