If I could ask you a question Kitru what are the defense stats, and stat weights I should be looking for?
While you're leveling, it doesn't really matter: the item rating matters more than what secondary stats are present, at least until you get into your 40s and start gearing for end game. At that point, until you start getting into the realm of top end optimized gear, rating still matters more than anything else, as long as it's got the right stats on it (anything def/shield/absorb is good; replace any mods with acc/power/surge with something with useful tank stats, even if it's got a lower rating). When you do get to the top end of gear optimization, you get into the eternal Endurance v. Mitigation debate (upon which I side firmly with "Mitigation"), but the same logic of "bigger numbers are better" before still applies in largely the same amount. The only difference is that, when you're doing the minute tweaks to your gear to eke out every bit of optimization, it gets a lot more complex than can be put simply thanks to the difference in relic choices (the passive heal relic, the proc abs relic, either of the use relics, and the passive PvP relics all mess with the optimal distribution in their own ways and all of them are viable to some extent in different situations). Personally, I prefer to maximize my shield through Enhancements (using the high Shield rating variants rather than the high End variants) and maximize while balancing Defense and Absorb through everything else (augs, mods, ear, implants, etc) while using a proc heal relic and a proc absorb relic (I would use a tanking use relic, but the 20 sec duration nerf to both of them at the DG tier kind of did them in for me). The simplest way, using top tier gear, was to just use a mk-1 ear, mk-2 implants, Robust 27 (unlettered) mods, and Bastion Enhs: it puts my Shield at 600, my Defense at 532, and my Absorb at 516.
If you're looking for a simple guide:
Leveling: Improve gear based entirely upon rating. Tank stats don't matter enough until you're actually tanking endgame.
Tionese-Columi: Just get them and get the set bonus. The tank stats are present, but the content you're doing in them doesn't matter enough to need to optimize them heavily.
Rakata: Replace the enhancements (since they have accuracy on them) with the enhancements from your Columi gear.
BH/Campaign/DG: Pay attention to the individual mods and enhancements and switch them out for the highest mitigation variants you can get. Armoring should be taken as they can be gotten. Maximize your shield while balancing Defense and Absorb (remember that you shouldn't use Shield augs because you need those for Defense and/or Abs).