I prefer the Vanguard/Bounty Hunter tank class, but many people seem to think it is a rather easy class to play. If you are looking fir diffuculty then I would suggest you try a Shadow/Assassin. They require quite a bit more concentration due to the various defensive cooldowns and such.

Guardian/Juggernaut are in a pretty strong place right now from what I understand. They are also the inbetween tank class. They are more passively survivable than Shadows/Assassins, but require more maintenance than the Vanguard/Bounty Hunter.

My personal preference would be to go either Assassin, or Bounty Hunter ... but thats just me, I never really got the feel for Juggernauts.

"What is the most useful tank?"

To be honest a player who takes the time and perfects their game will be equally useful no matter which tank class they pick. I would suggest starting with whichever one looks the more attractive to you and get them to the end of chapter 2. If you are not diggin the playstyle, pick a different one.