Disclaimer: The way that my ranking systems are set up is based specifically off of my personal opinion and their utility from a usage perspective in a traditional campaign setting. I view a traditional campaign as being one part roleplaying, two parts adventure and two parts combat. While your ability to roleplay based upon a particular selection will be taken into consideration, I always find that in DnD you can justify roleplaying specific ways despite your subclass. Some people may call that power gaming, but the point of DnD is to be challenged, have fun and stay alive despite the odds. If you die because you want to be cute and gimp yourself, you just failed on two of those. These ranks are also heavily weighted to abilities that are granted prior to level 15 as the vast majority of campaigns end before you reach that point.
Warlocks are a very interesting class that has a unique flavour and playstyle. You're not as adept at casting spells as a bard, druid, wizard or cleric and most of your turns will be spent casting eldritch blast. This can get a little bit redundant, but with your focus on charisma as a casting stat and the fact that you deal a solid amount of damage with your spells, you're valuable both inside and outside of combat.
Each of the Otherworldy Patrons very much change the way that you are going to play your character both in combat and roleplaying.
5: The Great Old One
If you're very creative or just want flavour GOO is for you.
The spell list you gain from this Patron is mostly situational with only a few standouts like getting Dissonant Whispers & Tasha's Hideos Laughter at level 1 (neither of which scale particularly well), along with the crowning achievement of Dominate Person. This subclass has a lot of flavor and situationally can be very useful in a roleplay heavy campaign or in the hands of a very creative player, but generally it's quite lackluster.
Entropic Ward at level 6 is the most flat out valuable thing you get, which allows you do impose disadvantage on an attack one per short or long rest. If the attacker misses, you gain advantage on them for your next attack.
4: The Archfey
You're druidic but better in nearly every way, with escape artist abilities.
Most of the spells that you gain through this patron are a bit lackluster though I am a huge fan of a free faerie fire at level 1. You immediately gain the ability with Fey Presence to potentially cause creatures within a 10 foot cube to become charmed or frightened by you. This is a great ability if you find yourself in melee combat for some reason, which you should not be with this patron. On that note, Misty Escape allows you to turn invisible and teleport up to 60 feet away when damaged in combat. Each of these are limited to once per short/long rest but they are surprisingly helpful as you are squishy as a warlock.
Your later skills allow you to counter-charm an enemy while giving you immunity to being charmed and completely remove a creature from combat and transport it into a misty realm.
3: The Celestial
Versatility expanded into being a very powerful healer.
I find this patron to be quite under rated in general. If you choose this path, you gain access to some pretty powerful spells like Cure Wounds, Guiding Bolt, Revivify, Wall of Fire & both Greater and Lesser Restoration. This turns you into a very versatile caster that can heal, eldritch blast and fire off some pretty powerful spells in general.
This drastically increases the value of your character particularly from the outset at level 1 with Healing Light by giving you a pool of D6 dice equal to your Warlock level +1. You can select any number of these dice up to your Charisma modifier to use as a bonus action on a turn to heal one creature you see within 60 feet. If you do not have a dedicated healer in your party this is what you should take to still be a powerful caster and face but also keep your party alive. It's just the responsible thing to do.
This Otherworldly Patron's additional benefits are a little lackluster in comparison but you do provide your party members with free temporary hitpoints and become a master of radiant and fire damage; gaining resistance to radiant damage and adding your charisma modifier to radiant and fire damaging spells.
2: Hexblade
You're now a very powerful melee combatant slinging spells.
Your warlock is now a viable melee combatant; not only viable, but a powerful one. You gain proficiency with medium armor, shields and martial weapons and you use your Charisma modifier instead of Dexterity/Strength as your attack modifier. This effectively means you can dump Dex & Str yet still be powerful in melee.
Nearly all of your spells you gain are based upon making you better up close and personal with combatants like Shield, Blur, Elemental Weapon & Wrathful/Branding/Staggering/Banishing Smites. You start out immediately powerful with Hexblade's Curse. Much like the vast majority of Warlock abilities, this can only be used once per short or long rest; but when you use it, your enemy will know. You gain bonus damage to one target equal to your proficiency bonus, you crit on both 19 & 20 (effectively doubling your crit chance) and if the target dies you regain hit points. Armor of Hexes at level 10 and Master of Hexes at level 14 only make you immensely more powerful.
Most people pick this patron because they want to multi-class into Paladin which is an incredibly overpowered build. Standalone it's quite good, however.
1: The Fiend
You're just a full blown caster now that gets rewarded for killing.
This Patron is set up to make you a magical wrecking ball, providing you with some incredibly powerful spells from the Wizard list including Burning Hands, Scorching Ray, Fireball, Wall of Fire and Flame Strike. In addition to this, you gain temporary hit points each time you kill an enemy – which you should be doing quite a lot as a warlock with this spell selection.
You also gain the ability to add a d10 to one roll per short or long rest to help it along with a chance to choose one damage type to be resistant to each time you short or long rest. This could be very valuable as upon a short rest you can adapt to the predominant damage type you're seeing in a dungeon.