I assume we are going for power-level here, or just overall usefulness anyways, in no order;
Tough
Heavy Armor Master
Crossbow Expert
Polearm Master
Magic Initiate
Tough gives you the hp of a +4 in con in one asi, can be great to get after that 20 con on a tank or instead of 2 +2 con asi on a meaty melee fighter, 16 con+tough is the same hp as 20 con and it saves you an asi.
Heavy Armor Master is great for tanking, a flat reduction to most incoming damage and a boost to your offense is crazy valuable. Fun exercise, tally up how much damage you reduced(or would have) over one adventuring day, it's surprisingly high.
Xbow expert and Polearm master are old standbys that I really shouldn't have to explain. But to put it shortly; getting an attack for your bonus action is real good.
Magic initiate really shores up a lot of classes shortcomings while not costing a multiclass. My usual suggestions are for Paladins to get thunderclap and firebolt because they have no real aoe or ranged options standard. Other good ones are for reducing MAD on Clerics and Rangers with Shilelagh, grasping vines is real good too. Honestly the level 1 spell could be totally removed from this feat and I would still use it just as much, the cantrips are the important part.