Creating a Fiery Spear wielder

[Haydensan]

I've recently become fascinated with the idea of making a character who's whole shtick in combat is whacking/stabbing things with a spear empowered by fiery magic.

I'll admit this is because i recently watched Fate/apocrypha and loved the character of Karna.

I know nothing will emulate it exactly but i was wondering what the best playgrounders could come up. All i've got so far is some sort of martial that has access to green flame blade.

[Arkhios]

I've recently become fascinated with the idea of making a character who's whole shtick in combat is whacking/stabbing things with a spear empowered by fiery magic.

I'll admit this is because i recently watched Fate/apocrypha and loved the character of Karna.

I know nothing will emulate it exactly but i was wondering what the best playgrounders could come up. All i've got so far is some sort of martial that has access to green flame blade.

Well, Eldritch Knight would be the best bet I'd say. Fighter is the most martial of martial classes, and Eldritch Knight gains access to Wizard spells relatively early. Alternatively you could take just about any other class with proficiency to spears and grab Green-Flame Blade through Magic Initiate.

Blade Pact Hexblade could also work if you'd prefer something more mysterious. Besides, assuming that that fate/apocrypha is some sort of anime, don't their prime protagonists pretty much always have some dark or mysterious power :P

[JellyPooga]

A Sorcadin with Green Flame Blade and Searing Smite ought to do the trick. Grab yourself some other flame-themed spells and you're set.

[Knaight]

Play literally any martial class capable of using a spear. Use a spear empowered by fire magic. Obviously this depends on the DM to some degree, but until the Mystic is out properly (or the Soulknife gets split into its own class) the closest you can get is someone who uses a spear and also fire.

[Mortis_Elrod]

Hexblade or Paladin, they both get elemental weapon and I think searing smite.

Forge Cleric’s get fire added on to weapon attacks at some point.

Besides that and some fire spells like Green Flame Blade and Flame Sword (Druid not as good as it seems) and extra attack/fighting style that’s about it.

Edit: actually just go Forge Cleric and pick up Green Flame Blade via race (half elf or elf of the high/sun/moon variety) or magic initiate feat.
You get searing smite, a +1 spear, resistance to fire, elemental weapon, green flame blade, and fire damage once per turn on weapon attack, all by level 8.

[Vaz]

If you're okay with homebrew, the Blood Hunter is the perfect class to use this with.

The Pike is a Heavy Weapon, so benefits from Great Weapon Master and Great Weapon Fighting Style, which the class provides, and the flaming bit comes from the Rite; you deal Character Level damage to yourself, and then add 1d4 damage to each attack with a chosen weapon, scaling up one dice size every 5 levels. One of the Blood Curses allows you to deal double Rite Damage, and even remove Resistance if you take Rite Damage to "Amplify it". Get someone dropping Hold Person every so often, and with at 5th level, you're putting out 2d10+18+8d6 (average 61) damage on a Paralyzed bromigo.

You can also pick up Order of the Mutant to become "the witcher"; Cruelty at level 11 gives you the ability to guarantee 3rd attacks, Guarantee 19-20 Crits, Up to +4 Strength Bonus, or between +4 and +10 to Initiative.

[Easy_Lee]

Edit: actually just go Forge Cleric and pick up Green Flame Blade via race (half elf or elf of the high/sun/moon variety) or magic initiate feat.
You get searing smite, a +1 spear, resistance to fire, elemental weapon, green flame blade, and fire damage once per turn on weapon attack, all by level 8.

I'd go with this unless you want whacking / stabbing things with a spear to be your only shtick. In the latter case, you could go Hexblade, Paladin, or EK and simply take supportive spells / invocations / features that help you do that better.

[tmjr6]

I would say the above options are some of the better non-homebrew options for you. Realistically, you could easily ask your DM for a spear Flametongue as well, when appropriate. If you want to pursue homebrew, there's always the blood hunter and my rave master class. The former if you like playing an eclectic monster hunter, the latter if you like playing a hero with a magic shifting weapon. Both provide options, the blood hunter's blood rites and the rave master's forms. You have a lot of options, personally I think the Forge Cleric might be the closest to what you are looking for.

[Grod_The_Giant]

Forge Cleric is a mighty good choice, but I'll also suggest a Draconic Sorcerer (particularly with a dip for either heavier armor or Dex-based spear attacks). Draconic Sorcerer is certainly the best at Green Flame Blade, what with +Cha to fire damage and Quicken Spell.