I don't think there's a "Best Avenger", but there is definitely some I prefer. Obviously if they all fought against each other, Hulk would probably win because he's overpowered. But yeah, here's my favs:
Storm. She flyes, shoots lightning, creates blizzards, intense rain, etc.
Hawkeye, agile sharpshooter with a funny attitude.
Wolverine, immortal badass with claws.
Spiderman, always has a one liner for every situation.
Black cat, better than Cat woman in every single way.
Hank Pym, just because he can become giant.
Iron Man, just for the attitude.
Thor, he drinks A LOT of beer.
Sif, she has a nice butt.
Wasp, she's a cutie.
Beast, I love the way he talks.
War Machine, just 'cause.
Dr. Strange, he says funny words.
Venom, he's just awesome.
Daredevil, he's a blind lawyer.
And of course, Kraven. Fucking beast. lol