just funnest to watch do their job well.
It depends on how you define fun/flashy.
For pure particle effects and special visuals, it probably goes to the Assassin: loads of purple lightning and black auras makes for a remarkably flashy visual style coupled with using a DBSD for twice the lightstick of a Guardian/Jugg.
For unique visual effects (for an assassin, it's pretty much lightning, lightning, and moar lightning as the flashy parts), it probably goes to the Powertech (if you like fire and knives) or Vanguard (if you like white-blue electricity and unintended uses for a blaster rifle coupled with explosions).
The least flashy tank, as I see it, has to be the Shadow. Instead of lightning, the Shadow gets rocks that, half the time, you can't even see because of other particle effects. Instead of dark auras, they get some more floating rocks (KW) and some slight yelllow auras that, when I first saw them, I wasn't sure if they were intentional or a graphical glitch. It still has the DBSD and some really kewl spinning kick moves, but those aren't going to play a major part in their tanking visuals compared to the rocks. Guard/Juggs at least get some jumping/acrobatics/explosive visuals to make them somewhat interesting.