A lot of tanks are extremely underpowered. There are virtually no exceptions to this, however
is doing well statistically because of gold funneling and synergy/countering with Slayers. In general, tanks are sub-optimal and not being played right now. Tanks cannot currently compete well in the jungle or top lane and they are barely competitive as supports.
A lot of mages are extremely underpowered. There are a few exceptions that are very strong, but mages remain very optional in general and maintain low total pick rates across the board besides the few exceptions which are still not major outliers compared to Marksmen, Fighters, or Assassins.
Many marksmen are very strong and are still in the vast majority of games. By most measurements, a marksman is still the strongest champion in multiple positions at any recent given time (
top,
bot,
bot,
bot,
bot,
jg,
jg are all the top pick in their position right now by different ways of measuring). A couple marksmen actually are underpowered (as is true for every other class), but they all remain relatively popular except
who is kept in the dumpster. As a class, marksmen are still probably extremely overpowered considering they are still in 90+% of teams despite both tanks and enchanters being out of the meta and assassins and divers being extremely prevalent.
Fighters in general are overpowered right now, though that may be in part because of slayers being picked in most games and some games having double slayers. If tanks were tankier, then fighters would be more balanced because they wouldn't win their lanes as hard and reliably against the few tanks still being played. They are still some weak fighters, like
and fighters are still considered less mandatory than marksmen according to pick rates.
Slayers (the primary class that includes assassins and skirmishers like

) in general are extremely overpowered, but especially AD slayers. Snowballing slayers have practically no counterplay at all and the ease at which they can snowball in the current meta due to the lack of functional tanks and enchanters to counter them is ridiculous. Slayers are just about as overpowered as marksmen right now, which is really saying something considering the meta of the game for the last 3+ years has had mostly mandatory marksmen.
Controllers (enchanters and catchers) are pretty balanced right now, but like is usually true, there are a couple (
) who are somewhat overpowered compared to their competition.
Buffing tanks' survivability without giving them damage, making mages more versatile instead of being burst or waveclear, and giving slayers more counterplay are all things that Riot should do. If tanks are buffed and slayers are nerfed, then marksmen should also get some compensation nerfs since they are statistically still doing well in a meta that is strongly against them, even if the role isn't much fun right now because of the slayers. Fixing the slayer meta will make things more fun for most marksmen players even if it costs them damage nerfs. If tanks come back and slayers become rarer without marksmen being nerfed, then the current 90% mandatory marksmen will go back to 100% quickly.