What should I pick from Chest of Loyalty?

Note: Due to changes introduced with the release of Heart of Thorns, the in-game economy will be unstable for a while and this answer is likely to change.

It depends on your needs / current goal

The choices you have are as follows:

1. A Vision Crystal and 1-2 of an ascended crafting material.

   Crafting a Vision Crystal yourself costs 20 spirit shards, 30 Obsidian Shards, and 2g26s for Thermocatalytic Reagent.

   As for the other mats, with some luck, you'll get Damask (worth 14g), but Deldrimor Steel (6g) is also possible. Assuming the chances of getting 1 or 2 are 50-50 and all mats are equally likely to drop, and using the current TP prices, this is worth 11g30s on average (after deduction of TP taxes).

2. 8 Obsidian Shards and 7 Mystic Clovers

   The former is easily obtained by farming the Silverwastes. The latter are required for the Gift of Fortune, which is necessary for crafting Legendary Weapons.

3. 20 Laurels

   Laurels are quite useful. Not only can you currently only get them via login rewards and achievement point rewards, you can use them to buy Ascended trinkets such as Rings and Amulets (Accessories are easier bought with Guild Commendations), as well as other stuff at Laurel Merchants. This includes Heavy Crafting Bags for 1 Laurel apiece, which contain valuable T6 fine materials currently worth 74s per bag, on average, totalling 14g80s.

4. 6 Tome of Knowledge
   These are abundant in PvP, and you got 10 of them through login rewards already. Unless you really need a new character boosted to 80 quickly, there are better options.

To conclude, Ascended Crafting Mats are a good choice if you are currently crafting ascended items, Legendary Crafting Mats are a good source for the mystic clovers. Laurels used to be the definite go-to material a while ago, but prices for T6 have dropped. They are still worth it if all you care about is getting your hands on your fellow players' money.