The idea of the 450 champions, aside from them being the first champions released in the game, is that they help new players get their feet wet in general roles and playstyles.
Garen is a manaless juggernaut that functions as a versatile fighter/tank.
Ashe is your basic ADC who helps understand last hitting and spell usage while focusing on basic attacks.
Tristana is your basic ADC who helps understand enemy focus (technically free, which also points new players to League's loudest mouthpiece so they can keep up on major news and events).
Ryze is a combo mage, which helps new players understand how spells interact with each other.
Nunu focuses a lot on farming with few champion-based spells (granted, he REALLY needs a rework).
Kayle is a self-sustaining mage fighter who teaches players that some fighters build AP over AD (though she might be on the rework table due to being an older champion)
Alistair is a basic tank. He also teaches new players about combos. (On this list because he's technically free)
Master Yi is your basic assassin, teaching players how assassins focus on a small window of power before their menace falls off tremendously.
Soraka is your basic support, emphasizing utility over her own firepower.
Singed focus on indirect warfare and careful positioning.
Annie is your basic burst mage, like Master Yi focusing on small windows of power and putting yourself at risk for the reward.
Poppy focuses on high durability and disrupting the enemy's movements and positioning.
Sivir is another basic ADC who focuses on crowd damage over a single target each time.
Warwick is a pursuing fighter who focuses on sustaining himself, locking down a single dangerous champion and chasing down weakened targets trying to flee.
Champions should be relatively equal in viability regardless of difficulty and IP cost, and for the most part, they are. It only becomes a problem if the enemy team starts overspecializing; Garen doesn't have any ranged ability, so if you keep him at range, he literally can't do anything, especially if he's constantly slowed. The last thing you want to do if Garen is fed or has marked you for death is to get in melee with him.
