D&D 5E Best 1st level feat for human warlock?

Inspiring Leader gives lots of temporary HP especially at level 1 and if you have a large party and no one else has the feat it is probably the strongest feat you can take at level 1.

Note that you can pick up medium armor + shields with a 1-level multiclass (I've seen a level in Cleric mentioned as a good multiclass in this regard).

Critical questions: which patron? What pact? Which invocations?

These matter, because for a Warlock they answer the most important question: what are you trying to do?

For example: if you're a servant of the Great Old One, Pact of Chains, Mask of Many Faces & Misty Visions infiltrator... then Actor is a very good choice for you. +1 Cha, ability to mimic voices, and advantage on rolls when duplicating someone.

So: tell us what sort of Warlock you're trying to build. What's this character's role?

Patron is custom. DM combining bits from each of the 3 in the book. So far it's all GOO (1st level ability & spell list), but that doesn't mean the rest will follow suit. The patron's name is 'The Lord of the Mists', & it's all about the power of truth - revealing it, concealing it, controlling it.

My character is going tomelock, & will be leaning heavily towards illusions & other forms of crowd control - to the point that I'll probably take Repelling Blast before Agonizing Blast (& Misty Visions is certainly 1st up on my Invocations list). I may take MoMF down the line, but not as one of my 1st two invocations; going to see how the campaign plays out a bit first, since we have another couple of CHA-skill characters.