Honestly, I wouldn't particularly choose a deck due to it being the 'best'. I'd choose the deck that has the most components in it which you'd use. Like, if you wanna play shadowcraft, choose the Nephtys deck so you could get a Demonlord Eachtar. Just choose the deck which has the cards you would use.
Which Pre-Built Deck is the Best?
Exactly as the title says. I want to know which deck to buy with my first 800 crystals.
Answer these questions and you'll probably arrive at a decision, somehow::
1- What format are you planning on playing?
(This will exclude Set 1 decks if you are focusing on Rotation)
2- What Craft are you planning on playing for awhile?
(If you are focusing on Rotation -- you're losing Albert from Sword, Daria from Rune, and Nephthys from Shadow in the Set 2 decks with the next expansion, if those are of any concern for you)
3- What Legendaries (or Gold cards) are you trying to get?
Just as a heads up if you haven't noticed already-- the pre-built decks aren't full decks (they are less than 40 cards) - so you will need to supplement the missing cards on your own.They also only have one copy of each Legendary when most decks will want to run x2~3 of those legendaries -- aside from maybe Aegis, but you'd be supplementing with x3 of Aether of White Wing to pull him out - so you will need to fill those in from crafting, card packs, or buying multiple packs, or finding a substitute for those cards. Additionally, some of the decks themselves have some rather questionable inclusions -- eg. Queen of the Dread Sea in what looks to be a PDK deck.
If you are a newer player, I'd hold off on buying pre-built decks until you've started getting some cards from your starting card packs so you'll have a better idea of what you might consider trying to build after you've pulled up some cards. It'll help keep costs down if you are trying to get one functioning deck that will allow you to start building up resources for making other decks.