Let me just start by saying that every card has its strengths and weaknesses and even if some card is objectively bad in most scenarios there are some cases when it can be quite strong.
The Nemean Lion
Why do I think that the Nemean Lion is the worst card in the game? Because of the mana cost. Don't get me wrong, having a creature that gets +1/+1 every time you play some other card with roar sounds great, but for 3 mana cost you basically get a 2/2 creature that is a sitting duck until you spend at least two more mana for two creatures with "roar" and get it to 4/4. If this scenario happens the card is not so bad, but the chances of having two 1 mana cards with "roar" in the same turn with the Nemean Lion are slim and not to mention that this does not make any strategic sense, usually 1 mana creatures with "roar" are played either at the beginning of the game or the turn when they are picked from the deck.
For 5 mana you can get a Highland Defender (4 mana) and a Felid Protector (1 mana) and have 3 creatures:
4/3
1/1 + Frontline
2/2 + Frontline
What can improve this card?
If the card had mana cost of 2, it would be overpowered, but if we can agree that the primary goal of this card is defense, then it would make sense to have 2/3 stats. In my humble opinion that would make it more balanced and worthy of being in a deck.
Do you agree?
GL HF,
Pure Gameplay