While Black Templars are primarily an assault army, some ranged shooting that can deal effectively with enemy armor is critical. Eradicators are my go to choice and I typically I run a five man squad with four heavy melta rifles and one multi-melta. That’s 12 shots due to their special rule Total Obliteration, which will typically trash most targets unless it’s got a good invulnerable save. Heavy melta rifles are always +2 damage and +4 damage at half range. This is extremely effective versus any target with -1 damage, as its basically ignored.
There are two tricks for Eradicators, one is giving them the Chapter Master rerolls, and great when they have to move to shoot or are shooting through certain cover (-1 to hit). The other trick is to use the stratagem Devout Push during your assault phase with them positioned so they move back towards an objective, taking them out of line of sight from enemy ranged weapons.
Eliminators with Lasfusils are another popular choice but I don’t rate them due to the low number of shots, plus they are quite squishy being a three man T4 squad with a 3+ armor save.
While dreadnaughts are typically an elite choice, they have many good options for ranged heavy weapons plus they are resilient and move-shoot well. The Redemptor dreadnought with a heavy plasma macro canon is a very popular choice - some players bring as many as two to three. The main drawback to the macro cannon is its d6 hits, which is extremely swingy. You’ll need at least two dreadnaughts to help endure you have enough effective fire power. I always arm my dreadnaughts with a dreadnaught close combat weapon so they can effectively deal with enemy units that charge them. Dreadnoughts have Eternal Duty for -1 damage plus there’s a stratagem, Wisdom of the Ancients, for them to provide reroll 1s to hit or to wound for core units, so are good to have around.
Another choice are Hellblasters but they aren’t nearly as effective as Eradicators. Gladiators are still overcosted and not core units either.
As a game progresses your melee units should be able to take out enemy armor with powerfists and thunderhammers - once you’ve reached the enemy lines if say your Eradicators are still in play it’ll be a tough choice for your opponent to focus on them, so it should be easier to find juicy targets for them to obliterate.