A general yet comprehensive list of everything the type matters for:
- Its type matchups (weaknesses, resistances, immunities)
- Miscellaneous defensive traits (damage from SR, immunity to poison, to paralysis, to powder moves, to prankster)
- The power of its STAB moves
The third means that although non-damaging movepool (like leech seed, light screen, will-o-wisp) don't change based on type, the damaging movepool
does.
Normal more than any other type has an extremely wide damaging movepool which makes its moves a good choice for this concept, but because its coverage is only neutral and never super-effective, whether or not the moves' user has STAB greatly impacts the effectiveness of its normal-type movepool. It's true -ate abilities can fill in this gap, but that already uses up the ability slot.
For
25w, the first two matter more, and anything decent there would work well.
Bug is weak to stealth rock and fire, but is otherwise good defensively (resisting grass and ground is valuable).
Poison is weak to ground, but is immune to poison and resists grass and fairy.
Steel is weak to ground and fire, but gives many resistances.
Fairy is much better on 25w than the other starters, because its damaging movepool is lacking, which hurts its participation in the concept for many abilities which affect damaging moves. Pure
Water is also a good option. Rock, Ice, Fire, and Electric, on the other hand, are all bad types for 25w. The first two because of their many weaknesses, Fire because of its specific weaknesses to Ground, Rock, and Water, and Electric because Electric and Flying especially are less common than Ground as coverage.
For
25f, the first and third matter more. It will spend a lot of time attacking, so STAB will be especially important. This means we want a secondary type, and it should be one with good
neutral coverage with fire. Water, Grass, Poison, Bug, Ice, Flying and Steel are therefore not recommended. Flying however deserves a second chance simply because it can switch in on many predicted moves, although its SR quad-weakness is a big deal. For reasons stated at the beginning,
Normal is a decent choice. However,
Electric also has a powerful movepool sharing many traits with Fire, and its coverage is better. Additionally, it gains resistances to electric and flying, which while not powerful in themselves, come only at the cost of overkill damage from ground-type attacks.
Dragon,
Rock,
Fighting, and
Ground are also good for coverage and movepool.
For
25g, as said before,
Normal is a powerful STAB (and much less powerful as coverage because it's not SE against anything), so is worth it, as well as being defensively decent (an immunity to ghost is absolutely worth a weakness to fighting). Being quad-weak to U-turn sets us up to have a whole lot of trouble with scarfers, so is not recommended. Grass resists two of the best attacking types, Ground and Water, which shouldn't be undone without strong reasons. Steel's many resistances are one such reason - if we want to make a generally good type and leave the direction-setting for later stages,
Steel is a pretty good secondary type. Otherwise, we've tentatively ruled out Dark, Psychic, Poison, Rock, Ground, Fire, and Electric. We could do Fairy, but Jumbao has a very broad role, and is also relatively stat-balanced, so we'd run into trouble competing with it. Ghost is possible, but has little that draws you to it, and Bug and Flying have unneeded resistances while their weakness to SR hurts.
Dragon is also attractive,
Ice even more so, but the type I like most is
Grass/Fighting, because of Fighting's good damaging movepool, and its resistance to both Ground and Rock, which carves out a defensive niche for it. Its coverage isn't as good, but that matters less for 25g because it's intentionally a specialist.
P.S. "I for one would like to be able to
do something about Necturna once I switch my Necturna counter in." If this is an issue, we can solve it with coverage. Necturna's bulk at -1 is not so powerful we need STAB to deal with it.