The GM plain to start a new game with us having to pick custom Race and starting at level 3.  I was think of going Artican with the new Armor sub.  This made me thing what feat should I take (can not take heavy armor master as it must be level 1).  I think I have narrowed it down to  War caster ( will still let booming blade on reaction) or Elemental Adapt Thunder since I plan to be in Gaurding mode attacking with Booming blade via gauntlets.  What do you all think and would Elemental Adapt even change the damage to a 2 since you did cast a spell but the damage if from the weapon?

Elemental Adept does not provide benefit to weapon attacks.  Only spells.

War Caster allows Booming Blade as an attack of opportunity, but as a tank you don't want to be fishing for Booming Blade's triggered damage.  You want creatures to remain stuck in place on you, attacking you.  War Caster is still good for retaining concentration and punishing creatures that move away, but you may wish to consider Sentinel instead, to stop creatures from moving in the first place.

You could also consider the new Fighting Initiate feat, especially if you intend to be a protector rather than just a Tough Guy.  Gaining the Protection or Interception fighting styles allow you to interfere with an enemy's attacks on your allies, putting your armored self in between them and harm.

It is a pretty even split between War Caster with booming blade and Sentinel if you want to lock down your enemies since both feats discourage or disable a creatures ability to move away from you. War caster will do more damage while Sentinel will allow you to hit them more often, but they don't really synergize that well with each other. In the same vein as these two feats, another feat you may want to consider is Mage Slayer, but you may want to wait a bit before taking it since it becomes pretty useless if you never fight spell casters.

If you want to increase the average damage of your thunder gauntlets, then I would recommend the feat Savage Attacker instead of Elemental Adept since it actually applies to weapon damage.

However, if you want more survivability (and you are playing a tank with d8 hit dice), then I would recommend Durable, Resilient (Dexterity or Wisdom), Shield Master, or Tough. Additionally, taking Fighting Initiate to get the blind fighting, defense, or interception fighting styles would be fun, but you need to have proficiency with a martial weapon which baseline artificers do not get.

Just talk with your DM first if you want to use the Warcaster/Booming Blade combo.

Booming Blade is cast on yourself/ your weapon now, while Warcaster requires you to cast a spell on the target.

Also note that Sentinal is a broken feat since it doesn't have a size limitation on it. Your DM may not be OK with you taking it, so check with them first.