PVE or PVP?

For PVE, most people run 5 Tavas Favor with Evasion (medium armor skill for dodge chance) so they can generate ultimate really fast to spit out warhorns. If just tanking 4 man stuff, I like to pair it with Dragon again for ulti regen and magic regen. In a really good group, I pair tavas with Alkosh instead for a debuff to boost DPS. Then you can add a monster set of your choice, depending on what you are doing. Warden is probably the best in trials, but there are other options, Bloodspawn, Grothdarr (more DPS), Engine Guardian (if you struggle with resources), Swarm Mother (chain in adds), Pirate skeleton (to really max out resist), etc. Some tanks run two one pieces to really max out their regen. A DK tank with infinite magic regen would be pretty much invincible.

Remember DKs manage their stamina with magic. Casting earthen heart abilities (Looking at you Igneous Shield), restores stamina, even while blocking. So most people dont necessarily stack a ton of max magic, but they want very high regen. This is also helpful if you want to cast chains a bunch of times to group up adds.