Which is better - HP or defense/sp.defense?

Question

Wouldn't HP sort of be defense and sp.defense because it doesn't matter if the move is attacking or sp.attacking?

Comments

it depends on  whether that specific pokemon has higher HP than Def/SpDef.

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I definetely think HP contributes to the bulk of a Pokemon, and in its own way, is kind of a defensive stat. But I don't know if it's better than defense\sp. defense.

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Percentage wise, the Defence and Special Defence stats are more helpful taking hits from their respective categories than HP is, but HP helps against both physical and special hits and is therefore more versatile. Which is preferable out of HP and a defensive stat as an EV investment is entirely situational; neither of them are "better" than the other per se.
In competitive play, defensively oriented Pokemon will generally invest EVs in both HP and their defensive stats, whilst bulky attackers that must invest in an offensive stat will often choose HP as it offer the most versatility. Fast attackers invest in Speed instead of HP or defences.

Answer 1

I personally disagree with the last answer. HP is, in a way, a mix of defense and special defense. Lets look at a calc real quick:

0 Atk Victini V-create vs. 0 HP / 252 Def Manaphy: 42-51 (12.3 - 14.9%)
0 SpA Victini Blue Flare vs. 0 HP / 252 SpD Manaphy: 42-51 (12.3 - 14.9%)

In this example, I have a blank set of Manaphy and Victini, but the Manaphy has 252 EVs in Defense and Special Defense (I also set V-create and Blue Flare's base power to 100 both to more evenly show the damage). Now, if we replace the defense/special defense EVs for HP we get...

0 Atk Victini V-create vs. 252 HP / 0 Def Manaphy: 54-64 (13.3 - 15.8%)
0 SpA Victini Blue Flare vs. 252 HP / 0 SpD Manaphy: 54-64 (13.3 - 15.8%)

(Same set up as last example.) The amount of damage Victini does to Manaphy increased compared to the defense/special defense EV spread. So in the case of leftover EVs, you usually want to invest in either defense or special defense instead of HP to cover a certain weakness, such as Garchomp's or Landorus's severe ice weakness. You could throw in a few leftover EVs into SpD to maybe tank ice beams from Vaporeon and such.

Back to what the question is saying however. To demonstrate this, I'm going to split Defense and Special Defense EVs so there are only 124 in each stat, and the useless leftover 2 EVs in each into the HP stat so we get only one extra HP point.

0 Atk Victini V-create vs. 4 HP / 124 Def Manaphy: 48-57 (14 - 16.6%)
0 SpA Victini Blue Flare vs. 4 HP / 124 SpD Manaphy: 48-57 (14 - 16.6%)

(We already calc'd max HP above, second example. Same set-up here.) As you can see, Victini's damage did even more than before.

To answer your question, I recommend to invest in one stat completely. Unless you know what you're doing and mess around with stat calcs a lot, I'd recommend going full HP EVs, Defense EVs, OR Special Defense EVs, instead of a mix of defense and special defense. I like to think of these EVs as kinds of 'multipliers,' in a way. HP EVs are like 1x multipliers, and Defense/Special Defense EVs are like 2x multipliers (NOTE: these are not exact figures and EVs do not actually multiply stats, its just my little way of remembering it). And technically speaking, it is smart to sometimes invest in both HP and a defensive stat to cover your other defensive stat's weakness with the extra HP.

Hope this helps. Ask if you have any more questions. :)
Damage Calculator.

Answer 2

No, HP isn't Defense and Special Defense. Defense and Special Defense reduce damage taken from their respective attacks. (Physical and Special) while HP allows you to have more HP left after taking damage. Here's an example.

Let's say a Pokemon of yours has 300 Max HP, and the opponent's Pokemon's attack deals 100 HP damage.
You can take 3 of those attacks before fainting, having 300 HP and taking 100 HP damage. Now, if your Pokemon had 400 HP, it could take 4 of those attacks before fainting. Here's another example.

Let's say you had a Chansey. (Just a normal average Chansey you'd get from the wild, no Eviolite, no Defense raising Nature, no perfect Defense IVs, no highest possible Defense EVs.) Despite having a TON of HP, it'd pretty much get One or 2 shot by a Physical Attack because of it's extremely low Defense.

So, basically, HP and Defense and Special Defense all have their uses and are all very important. But you need balance for a good Pokemon that can take a good few hits.

Answer 3

The simple answer is no, a Pokémon like Blissey couldn't take a focus punch with max defense, where a Shuckle would take it better. It also depends on what they are being affected by, a Shuckle could take a poison jab farely well, were Blissey could faint.