The DEX warlock is the best for PvP. By that I mean the 26 warlock, 4 shadowdancer build. No, not 27 warlock and 3 shadowdancer, and no, no hellfire warlock and don't even start dreaming about daggerspell or dragonslayer.
Just a pure 26 warlock, 4 shadowdancer, max DEX. The classics are required, practiced warlock, improved critical, blind fight, transient, eldritch mastery. The epic feats go to +D6 eldritch blast feats. All slots go to stealth. What you get is a high stealth character with irresistable damage, good reflex with evasion and passable will saves. This is the bane of most melee characters, especially with a wand of lesser breach to cancel energy immunity. It's fairly decent in PvM too, making you unlikely to die save for being too close to a wail. Use a deathward wand sometimes by the way. Around 105 damage per hit from what I can remember on average, plus acid damage over time etc.
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A similar focus is the CON warlock. I really like them, great for PvM, not too bad at PvP. Build them similarly to the above, with the usual classics. 27 warlock, 3 hellfire warlock, max CON. Same damage as above with the ability to get a bit more from hellfire, but for slots you focus on AC. You can probably afford improved expertise, even luck of heroes and a light shield with no ASF (frostmaiden's bane for instance). Steadfast determination is required. What you end up with is a pretty tanky character, my own CON warlock gets 51 AC and 485 HP for example, with excellent will and fortitude saves, and a respectable reflex. The high CON makes your concentration better, and the fortitude which is the main weakness of the DEX build, shines here. Plus, the amount of defensive bonuses makes you able to stand in melee and abuse the Undead bane blast or the repelling blast, making you fantastic at PvM.
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Then, the CHA warlock, focusing on DCs. It's a tough sell. See, the best DC you can get is a 26 warlock, 4 blackguard. Max charisma, prodigy, spellcaster, eldritch mastery and 5-6 epic charisma feats. That nets you a total of 19 base (10+9 for invocation) +1 (prodigy) +3 (CL) +13 (charisma) = 36+2 blackguard aura = 38 at close range.
Is that enough? Well, yeah. There are DC mages out there getting the same or less, but the best DC mages have you beat by about +6 DC. You've infinite uses though, but that only helps if the chance of success is there. Facing mind immune enemies can be a bit of a drain, as well as bosses, considering that your damage is around 75 on average. You've a slight advantage in PvP compared to the other two builds with eldritch doom, making you able to hurt druids, stormlords and some UMD users where they both are unable too, but it's a small comfort. DC warlocks are for the most part PvM focused, and most PvM content should be abusable with such a DC but watch out for the mind immune ones. Against them, it's all about slowing them down. Many melee and buffing characters ignore reflex, making it a fun build for PvP to walk up next to their ethereal behinds and just spam repelling blast over and over making them trip and wither away after a few rounds. Needless to say, focus on AC for slots, and maybe use that blackguard dip for divine shield too.
Verdict?
I've played all 3 at some point in time. I feel like I've had the most fun with the CON warlock, it has that nice blend of PvM and PvP power, and it's not overly reliant on something (enemy saves nor lack of stealth detection) making it durable. The CHA warlock has its charms (
), but for all the times you enjoy yourself slowing the enemy down or making them gibbering foolishly while they are taken apart by your allies, there are as many opportunities to feel lackluster in the damage department. DEX is a powerhouse waiting for the right PvP opportunity to shine, but that may never come and it's fortitude will get you at least once.
Maybe this is due to my love of Con focused spellcasters in nwn1, the conlock just seem to do it far better than those back then.