So there is no best tank for fp/wb/ops? Maybe u can describe few minuses of each tanks?
All of the tanks are perfectly functional with no major gaps, aside from potential PvP discrepancies (VGs are generally viewed as the worst PvP tanks because of their lack of survivability CDs, lack of utility, and lackluster ability to deal with F/T attacks). If there are appreciable cons attached to each tank class when properly played, it generally breaks down into how I described them before:
VGs have no real glaring problems aside from the fact that there isn't much that they can do to eke out superior performance; the only difference between a decent VG and a bad VG is that the bad VG is going to bottom out their ammo and constantly drop off their Ion Screen stacks but there isn't really much difference between a decent VG and a great VG. Their one major weakness, if it can be called such in the confines of PvE tanking, is their lack of strong utility functionality. Guardians have stuns and slows aplenty, and Shadows have CC, stealth, stuns, slows, and more in their utility belt. VGs really just have their pull, their leap, and a couple short duration, ammo eating stuns (barring the selection of some of the very, very bad slow talents in their skill tree that almost no one actually takes, that I know of). In summation, their weakness is their primary strength: they're simple so they don't have the same breadth of tools that can allow a well played tank to do truly amazing things.
The primary weakness of Guardians, once you've learned to play one, is that they have the longest recharge on their CDs, oftentimes forcing the Guardian to go through a burst DPS phase without a CD because they already used their CDs or expect a more pressing need somewhere further down the line. In a 6.5 minute fight, you can expect to be able to use your survivability CDs 1-2 times each whereas, thanks to the shorter CDs, a VG or Shadow could expect to use it 2-4 times. This can become something of a liability when a screw up in the middle of the fight that forces the tank to burn a CD ends up wiping the group because the Guardian didn't have their CD up during a high incoming damage phase. They've also got the worst threat generation and damage of all the tanks, and they lack the much desired and often lauded pull that VGs and Shadows get (though Leap>Push>Leap accomplishes something similar though takes 3-4 times as long and isn't as simple to execute). It's also important to note that Guardians make the worst off tanks: a lot of their Focus generation is dependent on being attacked and they rely upon Riposte for a not-insubstantial portion of their damage (5-10%), which can only be used after successfully defending against an attack. As off tanks, they're not going to be attacked that often and end up doing a lot worse in the role than either Shadows or VGs.
The major weakness of Shadows, assuming they're played properly and fight knowledge is present, is their hp variance. Because of their self healing and reliance upon active mitigation mechanisms coupled with high defense/shield/abs, as opposed to the strong passive DR that Guardians and VGs have, the health bar for a Shadow is going to bounce around a *lot* more than than of the other tanks. Once you're used to it, it's just part of the scenery, but many inexperienced healers get frazzled by it and aren't able to cope with it effectively, either dumping lots of overhealing on the Shadow in a waste of resources or underhealing in a waste of Shadow. This selfsame reliance def/shield/abs as opposed to DR also makes Shadows the worst tanks for fights that revolve around a lot of F/T attacks that deal K/E damage (since they have the lowest K/E DR and F/T attacks ignore def/shield/abs; for F/T I/E fights, on the other hand, Shadows are actually the best because of their self heals, Resilience, and the fact that they have the highest I/E DR). Also, Shadows do poorly in fights with a lot of attacks being directed at them in a short interval because of their reliance upon Kinetic Ward, which only provides the 15/20% bonus Shield chance against 8 attacks every 12 seconds. In general, this on affects trash fights (wherein 4-5 enemies attack them all at once, quickly depleting the charges, often in the opening alpha strike), which don't really mean much to most tanks, but it matters against bosses and fight phases with very high attacks per second (TfB tentacles and Foreman Crusher are the two that stick out in my mind as the only real offenders).
All of the tank classes have their little weaknesses and the answer as to which has the least weaknesses depends upon the fights in question, the player in control of said tank, and the group that they're running with. The Shadow weaknesses can be addressed pretty easily with an experienced player and healer (the KW "problem" isn't a major issue and the hp variance is more of a healer perception problem that is best dealt with through experience anyways), and the VG problems only really rear their head when you're attempting to push the limits (since neither a lack of utility nor the presence of a comparatively low skill ceiling is really going to hurt the average tank). Even the Guardian inability to offtank very well doesn't mean much since it's not like you bring a tank around expecting them to offtank as an important function (the only fights in the current top tier of content that really require an explicit MT/OT setup as opposed to tanks swapping or working together are the puzzle fights of which there isn't really high demand on tanks in the first place). A good raid leader will tweak who is tanking what based on the weaknesses inherent in the classes, but player skill matters more, honestly. The relative weaknesses don't really mean all that much in the hands of a tank who is good at their class.