Animancer's Energy Blade is top tier against certain enemies. Since it does raw it is never resisted by anyone nor is anyone immune. And versus enemies with very high pierce/slash resistance like say Dorudugan, or any of the dragons, a regular sabre user is going to struggle to do much damage unless they crit most hits (crit gives a +50% penenetration). Against regular enemies where you would penetrate most hits a regular sabre will do more damage, but the universality of animancer's energy blade is extremely useful. Same idea with Eccea's Arcane Blaster though I haven't used it as much. You can enchant both up to mythic. What changes is the weapons gain bonus damage and accuracy. With the blaster the imbued ammunition gains penetration since it does crush/corrode, and also you can add certain effects like interrupt/knockback chance and chance for bullets to split.
They're both mid to late game items though so you might think about importing them from another run if you want to use them the whole game. Just put the weapons on a character and go to export tab on the character sheet, then export. Then in another run you can import the adventurer at an inn and take the weapons. It can be very expensive if you give him too many items so I'd just give him the energy blade and the blaster. Or if you don't mind disabling achievements you can just add the weapons via console.
As Boeroer has indicated the builds that would benefit the most from these weapons specifically are ones with mediocre penetration and accuracy. When you do raw damage penetration is meaningless, and accuracy is not as important since crits don't affect penetration of raw damage. Classes with high penetration: devoted, berserker, cipher, monk (with fists anyway). Classes with high accuracy: rangers, ciphers, sometimes monks (if you can maintain enduring dance), and priests, fighters with conqueror stance and disciplined barrage.
Also a spellcaster class probably wouldn't need the sabre since they have spells, unless you built the class to be a martial caster like a wizard, who has many defensive buffs.
So...I'm thinking the classes that would benefit the most would be regular barbarians, rogues, paladins, monks, and maybe chanters and wizards. There are some interesting multiclass combinations there. A barbarian/rogue would be interesting thematically with the sabre if you take blood frenzy and blood storm, since you will do a lot of raw damage. Barbarian / streetfighter in particular would wreck things meleeing. If you want to use the blaster more then a SC paladin could be pretty strong in the right party, particularly a kind wayfarer since you can spam flames of devotion and heal allies with white flames, and at higher levels will gain back paladin resources whenever allies die (including summons from a chanter). Holy slayer (pal/rogue) is also interesting, particularly steel garrote, since it is easy for a melee character to trigger the healing passive. Bleak Walkers could work as well since persistent distraction + flames of devotion gives two afflictions and triggers deathblows. For rogue half if it's a melee character I'd be tempted to do streetfighter since it is easy to get flanked, but trickster has some nice defensive buffs. A blood mage / rogue using wizard side for defensive buffs and occasional debuffs to trigger deathblows could also be good.
I must note it's kind of weird to think about building a character this way, since normally I try to maximize accuracy and penetration, but they are just not as relevant here. Accuracy is still good, just not as essential, but penetration does nothing. Still the only classes where high penetration comes with large tradeoffs are berserker and devoted, so really I think you'd be fine with anything else...
I could give better feedback if you provided your other party members, what difficulty you're playing on, and the envisioned role for the character (DPS melee, ranged, melee/ranged hybrid, caster/melee hybrid, etc.?)