The summary of my answer is at the bottom. The following is mostly a breakdown of how well Darmanitan does in each of the games.
In both games, Darumaka is caught at the same place (Route 4/Desert Resort), so I won't be factoring in availability for either.
Gyms:
In Black, Darumaka/Darmanitan is caught before Burgh's gym and can be used for Burgh, Elesa, Clay, Skyla, Brycen and Drayden. Of these, Darmanitan can deal super effective STAB damage to the following Pokemon:
Burgh's entire team
Clay's Excadrill
Brycen's entire team
Compare that to Black 2:
Burgh's entire team
Clay's Excadrill
Skyla's Skarmory
You make a gym go from weak to Darmanitan, to strong against it in exchange for 1 measly Pokemon. So, in that factor, Darmanitan in Black is better than Darmanitan in Black 2.
Coverage wise, Bulldoze can help in Black 2 for Elesa more than it can in Black because of the 1 less Emolga, but that's it.
Elite Four:
For the Elite Four, nothing really changes between the 2 games.
Champion/Endgame fights:
In this regard, the 2 games are very different.
In Black, you face N and then Ghetsis. Darmanitan does SE STAB damage to:
N's Vanilluxe
N's Klinklang
Ghetsis' Bisharp
3/12 Pokemon, so a quarter of the fights. Not bad. With coverage, the following Pokemon are added:
N's Zekrom/Reshiram (Bulldoze)
N's Carracosta (Bulldoze/Hammer Arm)
N's Archeops (Rock Slide)
N's Zoroark (Brick Break/Hammer Arm) (STAB Sheer Force Flare Blitz does more but it's worth mentioning)
Ghetsis' Bouffalant (Brick Break/Hammer Arm) (Again, STAB Sheer Force Flare Blitz does more)
Ghetsis' Hydreigon (Hammer Arm/Brick Break)
*It's a risky matchup because the opponent has super effective damage against Darmanitan
As you can see, considerable coverage in the endgame fights. Compare that to Iris in Black 2:
No STAB SE moves.
Coverage wise:
Hydreigon
Archeops
Aggron
Lapras*
*See above
Endgame fight(s) are better in Black.
It should be noted that Colress in Black 2 is almost soloed by Darmanitan, so it should get some credit there. However, this isn't enough to oppose the fact that Darmanitan is better in more of the major fights in Black rather than in Black 2.
Synergy with your starter:
With Samurott in Black, the combo does quite well. Samurott's grass weakness can be dealt with by Darmanitan's STAB, and the electric one can be dealt with using Bulldoze. Samurott covers the Rock and Ground weaknesses of Darmanitan with its STAB, and resists Water.
However, in terms of synergy, Darmanitan has better synergy with Serperior, since all of Darmanitan's weaknesses are covered by grass, and the types that Serperior can't hit super effectively, Darmanitan can using its STAB or coverage. Overall, better synergy than with Samurott. However, I don't think this makes up for the worse performance in major battles than Black.
To conclude on which game Darmanitan does better in, I'd say Black due to the better matchups overall. For the alternative in Black 2, as the comments have said, Chandelure is a very good option.
Darmanitan is great in both games, but I will always defend Chandelure in BW2. Grinds easily against Audino, high Special Attack, strong TM pool, and great against the Elite 4. I think Darmanitan is a little better in BW and you should use Chandelure in BW2. -Amethyst
For a moveset for each of them, something like these:
Darmanitan @ whatever
Ability: Sheer Force
EVs: 100 Atk/100 Spe
Jolly Nature
- Fire Fang ---->Fire Punch ----> Flare Blitz
- Bulldoze
- Rock Slide
- Brick Break/Hammer Arm
Chandelure @ whatever
Ability: Flame Body
EVs: 100 SpA/100 Spe
Timid Nature
- Flamethrower
- Shadow Ball
- Calm Mind
- Energy Ball
If the answer is unsatisfactory, tell me and I'll edit/hide it.
Hope this helps.
