Can a creature with Defender still block?

Yes. Defender only prevents attacking; the creature can block normally and is often very good at it due to high toughness.

Can a creature with Defender attack if it gains another keyword?

Not unless an effect specifically removes Defender or says it can attack; gaining keywords like Flying does not override the Defender restriction on attacking.

What is the best Defender card?

By popularity, Defender is among the most-played Defender cards — see the full ranked list above.

Best Defender cards in MTG

Defender: A creature with Defender can't attack.

Defender appears on walls and utility creatures that excel at holding the line while your deck sets up. Many Defender creatures have high toughness or useful abilities like mana production, card draw, or pinging. In some strategies, effects can remove Defender or let walls attack, turning your defensive shell into a surprise offense.

FAQ

Can a creature with Defender still block?: Yes. Defender only prevents attacking; the creature can block normally and is often very good at it due to high toughness.
Can a creature with Defender attack if it gains another keyword?: Not unless an effect specifically removes Defender or says it can attack; gaining keywords like Flying does not override the Defender restriction on attacking.
What is the best Defender card?: By popularity, Defender is among the most-played Defender cards — see the full ranked list above.