Because most Pokemon increase their stats when evolving, you would not normally see a not-fully-evolved competitively. The most common exception to this is caused by pokemon holding Eviolite:
Effect: Raises the holder's Defense and Special Defense by 50% if the holder is not fully evolved.
It is important to remember that this will multiply not only base stats, but the final value, which includes IVs (Individual Values) and EVs (Effort Values). This is obviously most effective for acting as a wall. There are multiple pokemon which see competitive usage with Eviolite, but most notable are Chansey, and Porygon2. Dusclops, Doublade, Scyther, Golbat, Magneton, and Gligar may also use it well.
For numbers lets look at smogon's OU builds for Chansey and Blissey. Plugging the level (used 50, per VGC), EVs, nature and max IVs into a calculator (links on names below) we get:
HP Atk Def SpAtk SpDef Spd
Blissey: 331 27 68 95 187 75
Chansey: 326 22 62 55 157 70 (without Eviolite)
Chansey: 326 22 93 55 235.5 70 (with Eviolite)
This shows Eviolite Chansey has 137% Defense, and 126% Special Defense compared with Blissey, with 98% HP, 93% Speed, 82% Attack, and 58% Special Attack. This obviously increases survivability, and competitive Chansey would not have a move depending on Attack or Special Attack, so those stats would not matter.
A major downside of using Eviolite is that it is taking up an item slot, meaning no Leftovers or 'choice' items, but many competitions (including VGC) enforce an item clause (no duplicate items), which leaves leftovers for another member of the team.
Threads on smogon and gamefaq cover the eviolite users.
No. Effects that boost exp gain will not boost the entire parties exp gain when the pokemon in question participated in battle. You will see the active pokemon get boosted exp and then the rest of your pokemon get half of "normal" exp.
- For example if you had a pokemon with a lucky egg up front and he got
150 exp then the rest of your pokemon would get 50 (half of 100, the
original exp gain).
However, as Haidro points out, if the pokemon that would get boosted exp does not participate in battle they will still get boosted exp of the half they receive from the exp share.
- So if you had a pokemon up front with no boosted exp and they got 100. Most of your pokemon would get 50 but the one with the exp boost would get 75 (50 boosted).
Note these numbers assume 50% exp boost, I'm not positive that is correct but its easy to do math with and I feel it still shows the point.
Best Answer
Ditto excels at one thing in particular: scouting. Ditto allows its user to know exactly what moveset an opponent has without having to do any guesswork. This can be extremely valuable because several Pokemon can run multiple varied move sets and different roles. Determining a moveset is crucial to discovering what role a Pokemon plays in its team, and therefore how to defeat said team.
Ditto also does marginally well in countering sweepers by using their own stats and movesets against them. Dragon-types are particularly susceptible because they are weak to their own STAB attacks. If Ditto can get the jump on a Dragon-type sweeper, it can often take it out in short order even if you take boosting moves into account as Ditto copies stat boosts as well.
The problem with Ditto is that it spends one turn as itself because it needs to use Transform to adopt another Pokemon's stats. This means that any Pokemon that can outspeed it (which is several) can hit it with a super-effective Fighting move before it can Transform and possibly faint it before it gets started.
To use Ditto in competitive battles, you need support. This can come in the form of Wish, which can heal it in case it survives the initial hit; Reflect, which halves all physical damage; Light Screen, which halves all special damage; or any combination of the above or more.
Barring any support, leading with Ditto almost inevitably requires that you have its Hidden Ability, Imposter. Imposter allows Ditto to Transform as soon as it's sent out, without having to wait for you to spend a turn. This is crucial if you want to run Ditto as a lead or without support because that one turn is absolutely vital.
Also, note that while Ditto will copy stats including temporary stat boosts, Ditto will not copy base HP nor will it copy HP EVs. So, you will want to put at least 252 HP EVs if you're fielding a Ditto.