Each & every champ has it own special skill & ability.
It depends upon a player on what kind of champ he/she is looking for & what kind of play style they r looking for.
But, i personally like...

► Ash - The way he confuses the enemies making them run in different(dont know the exact name of the ability) & his pull is awesome !
► Galen - I love the way he roots the enemies & lands a punch on the face !
► Grimnir - The way he stuns with the continuous hits is so cool !
► Nova - Love her relentless shooting !
► Shard - His Decoy & teleportation using the decoy is simply Great !

I like to play each & every champ that i get for free as i currently just 6 or 7 but i still look forward to buy the champs which i love to play.