The sniffer 100%. Its so cute and it might bring a lot to the game. Also the other 2 options don't do much but I would pick the rascal over the Tuff Golem because rascals might give more then just iron pickaxes.

Voted tuff golem

Tuff Golem, hands down.

Sure, the Sniffer is irresistible to those who are into all things cute. And it gives a new plant to grow, but isn't a little too soon? We just got an array of plants from the Lush Caves.

And for the Rascal, an enchanted pickaxe is already simple to get, so I'm not looking for him in the dark. Even if the Rascal did have exclusive enchantments I wouldn't hold my breath. Some of Mojang's latest enchants added were only useful in certain areas, like the Soul Speed enchantment found in the Nether.

It realy depends in the implementation of the mobs.

Would be cools if the rascal would bring new rare enchantments.

And if the sniffer would find unique growable plans (that have a use in new recipes)