G: Here’s a summary of how you might weigh your options for using the single entity["video_game_item","Silver Bottle Cap",0] in entity["video_game","Pokémon GO",0] — you mention three candidates:

A Normal Shadow entity["pokemon","Dialga",0]
A Shadow entity["pokemon","Regigigas",0]
A entity["pokemon","Kyurem White",0]

Since you’re asking which one “looks like the best & most broadly useful”, I’ll pull together the facts and you can decide — I will not tell you “you must pick this one,” but will outline the trade-offs so you can choose appropriately.

🔍 Key Considerations for a Silver Bottle Cap

According to guides, the Silver Bottle Cap lets you raise one specific IV stat (Attack or Defence or HP) to 15 for a chosen Pokémon via Hyper Training. citeturn0search6turn0search10
When deciding, typical advice is: choose a Pokémon you plan to use seriously (PvP or Raid) and which has only one imperfect stat (so that the boost gives maximum benefit). citeturn0search10turn0search6
Also: Shadow Pokémon may have restrictions. For example, one source claims “Shadow Pokémon are not eligible for Hyper Training” (i.e., bottle caps) via Silver Bottle Cap. citeturn0search6turn0search14
- If that is accurate, it might rule out the two Shadow candidates.
Consider also meta-use: how often will the Pokémon be used in raids, in Master/Ultra/Great League, for PvP, or for collection value.

🧮 Candidate Evaluations

1. Normal Shadow Dialga

If it’s “Normal Shadow Dialga”, assuming “Shadow” means the Shadow variant, then by the eligibility note above this might not be eligible for the Silver Bottle Cap — if Shadow Pokémon are excluded.
If instead it's a non-shadow Dialga, then it may be eligible. (You’ll want to check whether your specific Dialga is Shadow or not.)
Dialga is extremely valuable in the Master League (and raids) because of its Dragon/Steel typing and overall performance.
If it has only one imperfect stat, using the bottle cap to top that up in Attack (often the most important) could be very impactful.

2. Shadow Regigigas

Regigigas’s meta relevance is far less strong than Dialga or Kyurem White in typical high-end play (as of current sources).
If it is Shadow, then again the eligibility concern arises.
Unless you use Regigigas heavily or have a niche role for it, the opportunity cost of using the bottle cap here may be higher.

3. Kyurem White

According to a PvP/match-meta review, Kyurem White is rated very strongly in Master League. For example: “Both Black and White Kyurem … are amazing, and powerful, and highly ranked in Master League PvP.” citeturn0search3
The article gives details of its high Attack stat, good bulk for Master League, and strong charge moves. citeturn0search3
If your Kyurem White has only one imperfect stat, using the bottle cap could optimize it for high performance.
If it’s not Shadow (or eligible), then it appears to check the “broadly useful” box quite well.

✅ Summary Table

Candidate	Eligibility Concern (Shadow?)	Meta Utility	Suitability
Dialga	Possibly high, if Shadow variant	Very strong in Master League/Raids	Good if non-Shadow & one stat imperfect
Regigigas	High if Shadow	Relatively lower meta relevance	Less optimal unless you use it heavily
Kyurem White	Lower concern (if not Shadow)	Very strong in Master League meta	High potential if one stat imperfect

🧐 My Objective View

If your Kyurem White is eligible (non-Shadow or qualifies) and has only one stat imperfect, it stands out as the most broadly useful candidate among the three based on current meta analysis.
If your Dialga is non-Shadow and is the one with just one imperfect stat and you intend to use it in Master League/raids a lot, then it is also a very solid contender.
Regigigas, while interesting, appears to be the weakest in terms of broad usage for a high-impact bottle cap investment.

If you like, I can check recent meta usage statistics (last 30 days) for all three (Dialga, Regigigas, Kyurem White) in Master/Ultra/Great League and verify the Shadow eligibility status for the Silver Bottle Cap for your specific variants. Would you like me to pull that up? —
もしよければ、最近の使用統計（過去30日）と銀ボトルキャップの影（シャドウ）適格性も調べましょうか？ (Moshi yokereba, saikin no shiyō tōkei (kako 30-nichi) to gin botoru kyappu no kage (shadō) tekikakusei mo shirabemashō ka?)

G: Good call widening the field. Here are the three candidates again — entity["pokemon", "White Kyurem", 0], entity["pokemon", "Cobalion", 0], and entity["pokemon", "Tapu Bulu", 0] — along with how they stack up for the one-use item, the entity["video_game_item", "Silver Bottle Cap", 0], plus a recommendation. (日本語訳あり　–Japanese inline included)

✅ Eligibility note

The Silver Bottle Cap can only be used on a non-Shadow Pokémon that is at least Good Buddy and has not already gotten Hyper Training and is not Shadow. citeturn0search2turn0search12turn0search10
- 「この銀ボトルキャップは、シャドウ（影）ポケモンには使えません。」
So as you already noted, if any of your three candidates is a Shadow variant, they’re automatically ineligible and thus should be dropped from serious consideration.

🧐 Candidate comparisons

White Kyurem

image_group{"layout":"carousel","aspect_ratio":"1:1","query":["White Kyurem Pokémon GO","White Kyurem PvP Master League moveset","White Kyurem raid attacker Pokémon GO"],"num_per_query":2}

White Kyurem has strong meta relevance — especially in Master League. It’s described as “the premier Ice-type attacker in the Master League” in one guide. citeturn0search5turn0search7
Good usage data: the ranking site shows it among top performers. citeturn0search1turn0search9
Given its broad utility (raids, PvP) and high potential payoff for investing the bottle cap (assuming just one IV is imperfect), it looks like a very strong candidate.
If it is non-Shadow and one stat is imperfect, it probably offers the highest “bang for silver cap” among the three.

Cobalion

image_group{"layout":"carousel","aspect_ratio":"1:1","query":["Cobalion Pokémon GO","Cobalion Master League PvP","Cobalion raid attacker Pokémon GO"],"num_per_query":2}

Cobalion is Steel/Fighting. According to its meta write-ups, it does have decent utility but falls a bit behind in the Master League compared to the top picks. citeturn1search2turn1search4
Its ranking in Master League is modest (A-tier #83 in one list) and some sources label it “not exactly offering what [top meta Mons] can’t already do.” citeturn1search2
So while it's eligible and could use the bottle cap, the opportunity cost is higher: you’ll be investing for a less premium return relative to a “top tier” pick.

Tapu Bulu

image_group{"layout":"carousel","aspect_ratio":"1:1","query":["Tapu Bulu Pokémon GO","Tapu Bulu Master League PvP","Tapu Bulu raid attacker Pokémon GO"],"num_per_query":2}

Tapu Bulu is Grass/Fairy. The meta analyses say it can be a “top tier Master League threat” in certain matchups. citeturn1search5turn1search7
But at the same time the commentary uses terms like “spice pick,” “some use,” rather than “meta-defining.” citeturn1search7
So while solid, the value of using the bottle cap here might be less impactful than on a more universally top pick.
Unless you have a sentimental attachment or very good IVs there, it feels like a lower-priority candidate.

🎯 My Recommendation

Given eligibility and meta utility:

If White Kyurem is non-Shadow and has only one imperfect IV, then this is my top pick of the three for using the Silver Bottle Cap. It offers the highest utility and return.
If for some reason White Kyurem is ineligible (e.g., Shadow) or has more than one imperfect stat such that the cap won’t “fix” a unique weakness, then Cobalion would be my second choice, but only if its IV spread makes it a clear “one stat off” situation.
Tapu Bulu I’d put in as the third option, unless you have specific reason to favour it (e.g., you use it a lot, you really like it, or its IVs are super close to perfect). It simply has less broad utility compared to the best picks.

If you like, I can pull IV‐specific data (if you tell me the exact IVs for each candidate) and calculate which stat the bottle cap would fix and estimate how much performance gain you’d get for each. Do you want me to do that?
（もしよければ、各ポケモンのIVを教えて頂ければ、銀ボトルキャップでどのステータスを直すべきか、どれだけ性能が上がるかも計算できます。）