The answer is always going to be to pick the one you like the most. If you enjoy the class and put in the practice, you'll excel. I myself prefer Juggernauts.
That being said, don't pick an assassin unless you're sure you'll be good. I can count the number of good assassin tanks I've met since 2011 on one hand. It's really weird that the most skill intensive class attracts so many bad players.
that's becasue holding threat on Sin is incredibly easy (plus stealth). but given that their mitigation is nearly all of active variety.. that makes them very challenging to play well
juggs incidentally are opposite. you are going to have to work very hard for your threat, but will be nearly unkillable. with some pretty awesome cooldowns to boot. - having played all three, in order of personal preference, I'd rate them: sin>powertech>jugg