Boozor here to go over my top 10 Arena impact cards which will most likely have the greatest influence on the Arena meta for the foreseeable months. Please keep in mind Blizzard intervention such as micro-adjustments or nerfs will obviously change the level of impact of each card. Also, I will be excluding Legendary cards from this list since Legendaries rarely affect the Arena in any substantial way (unless they can be readily discovered example: [Hearthstone Card (Dragonqueen Alexstraza) Not Found]) Keep in mind "impact" does not mean "best" - cards that are common will be encountered more often and their impact far more influential than an epic card for example.
#10 - Argent Braggart
Coming in at number 10, we have this insane Paladin card. Big minions haven't felt very safe against Paladin since Libram of Justice appeared last expansion, but this card punishes you in another way with a 2 mana huge minion. It's an epic so we won't see it often, but it's a huge potential threat and an auto pick for 2 mana often.
#9 - Reaper's Scythe
A unique weapon for Warrior that fills the 4 damage niche and has a powerful cleave effect - the issue is that it's a Spellburst ability. However, the fact that this weapon is on the board will force opponents into playing suboptimally due to the threat of cleave. It's just insane and will influence the board and how your opponent will respond to it.
#8 - Transfer Student
Very solid 2 mana minion with 10 different variants. Each version of Transfer Student is great and offers a wide range of utility. I'd expect this card to be picked very frequently.
#7 - Troublemaker
A large minion with a reoccurring end of turn effect. This minion is incredible on an empty board or to contest for it. It's better then Twin Tyrant in my opinion, and in some spots better then Coilfang Warlord. It's insane, and if Warrior has any chance at becoming Arena relevant again, this card will be at the center of it.
#6 - Fishy Flyer
Another very solid neutral common card that will be drafted very often. It's going to be another turn 4 card that can respond to an early lead or to build on your existing lead. We have Troll Batrider dealing 3, Burrowing Scorpid dealing 2 (and 5), and now Fishy Flyer dealing 4 damage. It's just going to be another card you will need to consider during your turn planning almost very game. The deathrattle gives you a bit extra value as well.
#5 - Plagued Protodrake
While this card looks pretty pedestrian compared to the other cards on this list - the fact that it's a common neutral means you'll run into this a ton in the Arena. With the existence of this card, it means every draft likely has a huge finisher that cannot be removed easily (transform effects will be most efficient, but only appear in 2 classes). The 7 drop leftover body has some variance (low roll 2/4, high roll 7/5 Cursed Vagrant), but you wouldn't want to voluntarily pop this to find out.
#4 - Onyx Magescribe
Similar to Plagued Protodrake, I don't consider this one of the best cards in the set, however, I predict that this card will be highly picked, and appear very often in almost every matchup. The body is very durable making the Spellburst more reliable to trigger. I'm a bit conservative about the whole Spellburst mechanic in general, but this is a nice body, has a Dragon tag, and decent value upside if you can pull off the spell trigger.
#3 - Cycle of Hatred
One of the best overall cards in the set - and a rare at that. The swing from this card is just insane, and for a powerful AOE card, you're actually pretty happy to draft multiples of these. Remember how strong Abyssal Enforcer was? It left 6/6 worth of stats after destroying the board with a 3 damage clear - this card will likely leave more than 6/6 worth of stats.
#2 - Blessing of Authority
Incredible card from Paladin which makes any minion on the board scary. This card will surely change the way you approach the Paladin match up. Anything on the board is a huge threat as long as this is in rotation.
#1 - Smug Senior
A 5/7 taunt offers very reliable stats for the mana cost, and the deathrattle gives you additional value. Considering that there just aren't many taunts available in this meta, this is going to be picked fairly often, so be prepared for 14 hp worth of taunts in any given game.