Also fuck Teemo
Seems like you already got the basics down
To advance, fuck Gnar(teemo 2.0) aswell.
To the more serious things:
i mean is there anyone who is indisputably too good right now.
Depends alot on what level of play you are looking at and how experienced the player is with said champs. For low skill levels(asumeing this affects you the most as you're still new to the game), champs like Akali, Darius and Yi usually are rather dominant, they are rather easy to use and punish mistakes really hard, but at higher levels, coordinated teams can abuse their weaknesses.
What types of teams are really common right now and is the meta set in stone?
There is not really a specific type of team that is really common for lower levels of play, but the rough meta is set in stone: You got a sololaner top(usually bruiser or tank), A jungler(usually tank, bruiser or assasin), A sololane mid(usually mage, adc or assasin) and a duolane bot with support or tank + ADC.
Does the insane amount of playable heroes in LoL mean that it'll never be the case that only a handful of champions will be viable?
Sadly not, atleast for high level play you usually have a few heroes per role that are toptier and some that get used by their diehard mains, while the majority hardly gets used.
However, in lower skill levels you can usually be succesfull with about any hero if you put in some effort to master him.