I have a theory that actually Gollum long term was the strongest of those that ever carried the ring. hear me out. sauron isn't influenced by his own ring just made more powerful so he does count. out of all the owners of the ring gollum had it the longest for over 500 years (other than sauron). Ok yes the ring very easily took hold of smeagol and could influence him to do horrible things like murder deagol. but i would argue that his duality the gollum and the smeagol side are actually smeagols way of fighting the rings influence (gollum being the rings influence and smeagol being who he naturally is) Ok some times they work together, and some times they fight (notice its always gollum trying to persuade smeagol) but the fact that the ring didnt take him whole is quite impressive and actually shows that the ring couldn't ever fully conquer smeagol in all that time because a part of him was still good and honest (if again easily influenced) so it could be said that he resisted the ring for over 500 years which is very strong. no doubt he was connected to the ring and as they said would never be rid of it but he wasn't taken he wasn't a wraith even after all that time. when you compare that to the nazgul all whom were taken by lesser rings in a much shorter time period. even if you look at frodo, he was nearly taken by the ring in just under under a year that he carried it (ok the ring knew he was trying to destroy it but the point still stands) Bilbo got off lightest actually because ,ok he struggled to give it up but did he carried it for 60 years with very little ill effect. so out of all the people influenced by the ring by his resistance over 500 years i must conclude smeagol was the strongest.