Everyone Seems to forget a few things....
1) Palps is the Strongest sith ever because he mastered all Sith forms, traditions, and evolved the Sith Tradition. Palps killed 3 yes i said it 3!!! of the best Lightsaber duelists in the Order under Yoda and Windu...Obi-Wan is up there but against Palps he wouldn't last as long as Plo Koon, and Sasee Tinn....also remember Palps fought both Windu, and a fully powered Yoda and survived because he is smarter than them.
2) Anakin vs Obi-Wan was one of the most dangerous fights for Obi-Wan....Obi admitted he couldn't fight Anakin in a straight duel because of Anakin's raw power and technique. Obi had to use his experience against Anakin's lack of emotion. Anakin over powered, and was too quick even for Obi-Wan's experience so Anakin can easily defeat Obi-Wan
3) Luke vs Vader was a one sided fight first - Luke is too inexperienced to fight Vader on equal footing i don't care what anyone says, Luke can't overpower Vader, and also has no dueling experience like Vader...and second Vader wasn't fully into the fight he didn't want to fight Luke his son, and when he found out about Leia as he daughter he realized his true legacy and tried one more push against Luke. If Vader wanted to he could have easily crush Luke with so much quickness...Remember Vader fought for 2 years in the Great Jedi Hunt, killed many jedi's and whole armies!
The Strongest Sith is Palpatine - Knowledge = more power no matter what.
The Strongest Jedi is Luke/Anakin - Depending on experience