2.49 BiS
2.5 GCD BiS
What to wear before getting savage BiS?
The above gear set is only an example, there are tons of options!
6.3 introduces two new sources of 620 gear and a new 625 weapon. As such it is extremely easy to get at least ilvl 620. Because there are now so many options, pick what is easiest/most affordable for you to get. When choosing between items of the same item level, prioritize items that do not have skill speed where possible. Remember when spending tomes to buy BiS tome pieces first. Upgrade the tome pieces with the Alliance Raid tokens and fill in the rest of the pieces with 620 augmented crafted or Alliance Raid pieces until you can get the BiS pieces. If you are not doing savage, just grab the highest ilvl pieces available, which will mean eventually getting all tome pieces and upgrading them to 630 using Alliance Raid tokens.
6.35 update: 630 Amazing Manderville weapon is better than the 630 Augmented Lunar Envoy weapon, but is not better than the 635 Abyssos weapon due to item level.
If you want to know specific differences between pieces, consult the Gear Planner as it will heavily depend on what the rest of your gear is!
Dragonsong's Reprise (Ultimate) BiS (6.3 Dungeon Gear Option)
If using the scythe from DSR, meld two Direct Hit X’s and one Determination X. If you want full compatibility with DRG after acquiring the DSR weapon, change the SKS meld on the head piece to DET and then do DH DH SKS on the weapon.
Note: The 615 and 630 versions of the Manderville weapons sync identically in DSR and are therefore the same. Use whichever you have available.