The Master of Elements

Style of Play:
In PVP you are a wonderful striker and a 'solid debuffer' (thanks to Ray of Enfeeblement); you will deal Very High DPS (more than any other path, though Renegade can be a contender if provided with a Losmauth set) and will cast your encounters at a very high pace, so it's the ideal path for PVP. All it's tuned to achieve the best possible DPS.

In PVE you still deal Very High damage, more if you focus on just burst damage (through Arcane Presence+Chilling Presence) but that's not advisable since you aren't buffing your allies through any feats like a Renegade would do instead. With this build CC is of secondary importance but obviously you can still control mobs well, the same as a Renegade build, or just a bit better. By choosing a build focused on personal DPS you lose the CC special qualities of an Oppressor and the ability to buff allies proper to the Renegade tree but you will still retain a specific feat from the Oppressor tree that will let you bring some more CC to the table thanks to Twisting Immolation.
In a party session you will want to help more your party by debuffing mobs and the boss through the combination of Critical Conflagration+Swath of Destruction; that's not the case in PVP where you want to boost just your burst damage. If you don't want to renounce to the debuff aspects of a Master of Flame but still want to bring solid damage without the employement of the Lostmauth set this is the path you will want to follow.

DPS:
Very High on everything, less if you slot your debuffing abilities that can be generally reserved for Boss fights or tough runs.

Damage:
-5%~ of my Lostmauth Renegade build's damage, good for those who don't want, or can't invest on the OP Lostmauth set.

CC:
High in PVE (just a bit more than my Lostmauth's Renegade build thanks to the Feat Twisting Immolation; it will let you Daze briefly your foes after a cast of Furious Immolation), Medium in PVP.

PVP:
Very High performance.

Buffs:
None.

Heroic Feats:
Controlling Action (1), Weapon Mastery (3), Toughness (3), Fight On (5), Blightning Power (3), Learned Spellcaster (5), Focused Wizardry (3).

Thamaturge Feats:
Tempest Magic (5), Snap Freeze (5), Elemental Reinforcement (5), Spell Twisting (5), Frozen Power Transfer (5), Elemental Empowerment (5), Assailant Force (1).

Oppressor Feats:
Bitter Cold (5), Twisting Immolation (5).

Powers (PVE, both party and solo):
At-Will - Chilling Cloud (necessary to achieve best possible DPS) (rank 3)
Encounter (Tabbed) (1) - Conduit of Ice (very high AoE damage, it also adds Chill stacks and will slow down your foes) (rank 4)
Encounter (4) - Ray of Enfeeblement (debuff for big mobs and bosses) (rank 4)
Encounter (2) - Icy Terrain (waste of DPS but necessary to control the battlefield, it briefly immobilize and can freeze targets + adds Chill stacks) (rank 3)
Encounter (3) - Steal Time (CC plus good damage against multiple mobs) OR Chill Strike (modest CC + Chill stacks) OR Entangling Force (good CC against single targets) (3/4)
Class features (option 1) - Critical Conflagration+Swath of Destruction (less personal DPS but 19% more DPS for all other party members) (rank 4)
Class features (option 2, better for solo play) - Arcane Presence+Chilling Presence (more personal DPS but lacking party debuff, except for Ray of Enfeeblement) (rank 4)
Daily - Furious Immolation (solid damage + AoE CC in a large area with an added Daze effect as a bonus) (rank 3)
Daily - Ice Knife (high single target damage + Prone effect) (rank 3).

Powers (PVE, Boss fight/Demogorgon):
At-Will - Chilling Cloud OR Scorching Burst (the latter just for Demogorgon's 1st. phase) (rank 3)
Encounter (Tabbed, option 1) - Shield (rank 3/4)
Encounter (Tabbed, option 2) - Conduit of Ice (to use in Demogorgon, 1st. phase only) (rank 3/4)
Encounter (1) - Ray of Enfeeblement (debuff for big mobs and bosses) (rank 4)
Encounter (2) - Disintegrate (rank 4)
Encounter (3) - Chill Strike (rank 3/4)
Class Feature - Critical Conflagration (boost DPS and spread Smolder on Crits) (rank 4)
Class Feature - Swath of Destruction (19% debuff for entire party, a must in boss fights) (rank 4)
Daily - Arcane Singularity (spam AoE CC in a large area) OR Oppressive Force (spam AoE CC + an added Daze effect in a very tight area) (3)
Daily - Ice Knife (high single target damage) (rank 4).

Powers (PVP):
At-Will - Chilling Cloud (rank 3)
Encounter (Tabbed, option 1) - Ray of Enfeeblement (rank 4)
Encounter (Tabbed, option 2) - Shield (if geared and with all PVP boons) (rank 4)
Encounter (4) - Disintegrate (rank 4)
Encounter (1) - Entangling Force (rank 3/4)
Encounter (3) (option 1) - Chill Strike (more DPS, brief CC, add a stack of Chill) OR Repel (far more useful CC against weak targets) (rank 3/4)
Encounter (3) (option 2) - Ray of Enfeeblement (if geared with Shield on Tab) (rank 4)
Class Feature - Arcane Presence (rank 4)
Class Feature - Chilling Presence (rank 4)
Daily - Ice Storm (Knock + Prone multiple targets) (rank 3)
Daily - Ice Knife (rank 4).

Advice:
Boost INT (DPS bonus, Recharge Speed Increase) and CHA (Critical Chance and Combat Advantage Damage) the same, ditch everything else.
You want to focus on Power, Critical Strike, and Armor Penetration in that order, but if your Critical Chance is lower than 34% invert Power and Critical Strike priority, everything else is of secondary importance.

Equipment:
Lostmauth's whole set, Vorpal Enchantment, Dusk plus Drowcraft armor set (PVE) OR Grim/Burning armor set (PVP), Ring of Brutality, Drowcraft Assault Ring, Savage enchantments in Offense slots, Radiant OR Dark (if you can gather enough Life Steal to benefit from it) enchantments in Defense slots, Lesser Soulforged Enchantment (PVE)/Negation Enchantment (PVP), Lantern of Revelation (main), Sigil of the Controller OR Wheel of Elements (the latter can be chosen as main instead of the Lantern), Sigil of the Great Weapon Fighter.

Boons:
Sharandar - Elven Ferocity, Elvish Fury (PVE)/Elven Resolve (PVP)
Dread Ring - Shadowtouch, Rampaging Madness (PVE)/Endless Consumption (if you got enough Life Steal) OR Augmented Thayan Bastion (PVP)
Underdark - Drow Ambush Tactics, Dwarven Stamina
Tyranny of Dragons - Dragon's Thirst
Icewind Dale - Cool Resolve, Winter's Bounty.

Races:
Dragonborn (+4 to any Ability Score, +3% to Power and Critical Strike) and Tiefling (+5% DPS against targets with less than 50% HP, rare chance to gain a 5% damage debuff against an attacker when you get struck) are the best races, buff aren't big so pick the one you like in the game you will have to see its traits everytime.

Enjoy.