To you should ABSOLUTELY use Giratina.
Giratina’s forms both have incredibly high stats. While Altered is more bulky, Origin is preferred for higher attacking stats, which is more useful in the game.
As for what to replace, I suggest Bibarel. It has type overlap with Staraptor and (more importantly) Floatzel, and has poor stats compared to Giratina. For a comparison:
Giratina
HP: 150
Attack: 120
Special Attack: 120
Defense: 100
Special Defense: 100
Speed: 90
And…
Bibarel
HP: 79
Attack: 85
Special Attack: 55
Defense: 60
Special Defense: 60
Speed: 71
Giratina is clearly superior stat-wise.
As far as movepools go, both can learn an impressive amount of TMs, but Giratina is better in this respect. Giratina learns strong STAB attacks like Shadow Force and Dragon Claw by level-up as well.
Now, you likely have Bibarel for the amount of HMs it learns. You could still use withdraw Bibarel from your PC to use HMs. It is also worth noting that Giratina learns every HM that Bibarel learns except for Surf and Waterfall, but with the addition of Fly and Defog. It is also worth noting that Floatzel can learn both Waterfall and Surf.
Finally, typing. Giratina’s Ghost and Dragon provides spectacular neutral coverage, and provides immunities to Normal and Fighting, as well as Ground with Giratina-Origin’s ability levitate. It does overlap with Garchomp, but dragon typing is very strong and useful. Bibarel overlaps with multiple team members, as previously mentioned. Water and Normal provides OK coverage, an immunity to Ghost, and weaknesses to Electric (Volknar’s gym, which Giratina happens to resist), Grass, and Fighting. It is worth noting Giratina’s weaknesses to Ice, Ghost, Dark, and Dragon, although these types are all rather uncommon and it is also super effective against Ghosts and Dragons.
