Regarding the Race, I consider Eladrin and Shadar-Kai to be among the top choices nowadays due to their free teleports which aren’t considered spells (which means they do not interact with stuff like Silence, Counterspell or being unable to perform spell components for some reason - and they do not interfere with your spellcasting, meaning you can teleport and cast a leveled spell on your turn). If you are a purely ranged build though, you likely won’t need to use them as often as a melee build.
Fairy and genasi (partiuclary for Genie warlocks) are cool for flavor.
Fallen Aaasimar work great thematically as Undead warlocks, while Scourges and Protectors make good Celestial warlocks, at least in terms of flavor - they unfortunately have quite some overlap in features. The old aasimars from MTF transform as an action but deal significantly more damage, while the new ones from MotM transform at a bonus action (that might actually conflict with Form of Dread and spells like Hex, hence going for the old aasimar might be better), but deal considerably less damage.
But for the most POWER, I’d suggest starting with Custom Lineage, buying a 17 for Cha, bringing that to a 19 with your race and taking whatever feat you’d like - good ones are Warcaster, Metamagic Adept, Moderately Armored (unless you multiclass or go Hexblade for some reason), Cartomancer, Rune Shaper (three free spells per day on top of comprehend languages). At level 4, you can then pick a Cha half-feat like Fey-Touched (hey, there’s your free Misty Step!) and bring Cha to 20 with it. That way you have a 20 Cha stat, a feat and a half-feat by level 5.
Regarding the patron, both Genie and Undead are great choices - it depends on who you imagine the character to be. Both can be reflavored too, if you want - a Genie warlock can be fey-flavored (fairy wings/dust for flying, association with e.g. the winter court as a winter eladrin) for example.
Undead is particularly nice with Repelling Blast, as a feared enemy cannot move back after they got pushed away.
Genie is great all-around for utility - you get flight at level 6 and lesser wish at 14.
Assuming you use Eldritch Blast for attacking, a Genie warlock deals slightly less damage than an Undead one in Form of Dread until level 17 - 1d10 averages 5.5, your proficiency bonus increases to 6 at level 17. That means for most of the time, Undead deals more damage. Also, as the extra d10 is doubled, you always deal more damage on criticals as an Undead warlock.
In terms of builds, the Genie (Dao) Warlock is notable for getting Spike Growth - if you take Metamagic Adept (or do a sorlock multiclass), you can quicken Spike Growth and immediately start eldritch blasting, cheese-grating the enemy. Since Dao adds bludgeoning damage, you can even go further and pick up the Crusher feat for an additional 5 feet push.
In regards to invocations, you want to have Book of Ancient Secrets as a tomelock for ritual spells, and Repelling Blast. I actually consider Repelling Blast over all better than Agonizing Blast. Of course getting both Agoizing and Repelling Blast is a good choice once you hit level 5.
Devil’s Sight is very niche unless you want to go for a Darkness build, but that always risks obstructing teammates (technically, they would still attack with straight rolls, but not many know that and being blinded feels restricting even if it does nothing at the end. Also, certain characters love attacking at advantage and getting attacked at disadvantage - Ardurion looks at you 
Another invocation that many warlock players enjoy is Misty Visions for Silent Image at will. Can open the doors to tons of illusion shenanigans, assuming the DM is ready for that 
Finally, if you enjoy eldritch horror, you want to take a look at Gaze of Two Minds. It is very niche, but when you can get it to work, it is awesome.
If you need more spells to scribe, you can always ask the professor 
or Blackie 
- they have tomes you can scribe from if you want to either risk your sanity by reading things that should not be read or if you like to scribe your spells as metal songs.
If you still consider multiclassing, some combos that I really enjoyed mechanically and thematically were:
- Genie Warlock / elemental-themed sorcerer (or wizard, that was with a houserule for Int-based warlocks though) - works great due to the bonus damage the warlock gets on EB being of your element, and you can nicely flavor your vessel accordingly - such as as a snowglobe if your character is winter-themed).
- Undead Warlock / Conquest Paladin - all about fear, being a dread knight who rules the battlefield with violence and cruelty - are you secretly a Bhaalspawn thirsting for blood, can you control your urges? Funny detail: by using Form of Dread, Wrathful Smite, the Menacing Attack maneuver and the Knight of the Sword feat, you can force an enemy you hit with an attack to make up to four Wisdom saves or be frightened…
- Fathomless Warlock / Aberrant Mind Sorcerer - probably the best thematically for an eldritch horror character, although the tentacle loses some appeal once you get more sorcery points to burn on quickened spells.
- Celestial Warlock / fire sorcerer (in my case that was an UA Phoenix sorcerer, but Divine Soul or others work too) - you are the zealous sacred flame that lights the dark and cleanses the world from evil.
- Hexblade / Paladin - I know that is not what you are going for, but it simply works too well together to not mention in a section about multiclassing warlocks…
xD
I havn’t used custom lineage so far, I shy away from all things giving off the OP vibe, that’s also why my cleric ended up being of the grave domain instead of twilight, plus I really like the flavour 
