Unfortunately, Skyrim was designed in a way that did not accommodate all races. This list includes a majority of races that were made worse by Skyrim. In Oblivion, these races were balanced well to make them viable, but Skyrim was designed for a different build, and these particular races don't work very well.
This isn't a case of bad races, just an area that doesn't match them.
Nord
Even though it's the best of the worst, it's incredibly popular. Due to their popularity and that "Skyrim belongs to the Nords," many players will choose Nords. Nevertheless, they have one of the worst race skills, which is basically a bad version of the fear spell.
They aren't as strong as Orcs, and they aren't as skilled as Redguards with weapons. Additionally, they both have bonus points in Two-Handed, Block, and One-Handed skills, which contradict each other. Overall, this is a bad race.
Dunmer (Dark Elf)
Dunmer were a great race in Oblivion but so terrible in Skyrim. In the early stages of the game, the Dark Elves are pretty powerful and interesting to play as. As you level up, Ancestor's Wrath loses all of its power since it deals a static amount of damage that doesn't scale.
Their Magic isn't as good as that of the High Elves or Bretons, and they can't fight as well as other races. If the others weren't so terrible, they'd be the worst on the list.
Related: Best Races to play in Skyrim
Argonian
One of the races in Oblivion that was great, but terrible in Skyrim. Skyrim's breathing underwater ability is almost completely pointless, unlike Oblivion's water levels and acrobatic skill. It is also useless that they have increased Disease Resistance since you can quickly cure yourself of them.
Hitskin, their racial skill, is the only good thing they have as they can heal themselves in an emergency or tank heavy blows from powerful enemies.
Bosmer (Wood Elf)
One of the coolest racial abilities of the Wood Elves is Command Animal, which allows them to command wild animals to fight for them. As you will rarely find animals in cities, lairs, and most dungeons, so their racial ability becomes useless quickly. As soon as you find potions to combat passive disease and poison resistance, their resistance becomes futile.
A Wood Elf's skill as an archer was much better in Oblivion because the areas were far more open. Unless you're fighting a dragon, you're inside of some dungeon in Skyrim. The Dunmer and this build were my favorites in Oblivion, but I did not like them in Skyrim because they were useless.
Khajiit
Khajiit suffered the most in the change to Skyrim. Skyrim lacks the unarmed skill, which was an excellent part of Oblivion. With the basic skill obtained after trying to enter the Mage's School, seeing in the dark is incredibly easy.
In addition, their ability to brighten the screen is useless since you can do it in the settings. This race became obsolete.
For more information on Skyrim, check out Best Followers in Skyrim and Best Mods for Skyrim on Pro Game Guides.