The 17 types are:
1. Steel
2. Flying
3. Water
4. Psychic
5. Dragon
6. Electric
7. Fighting
8. Bug
9. Ghost
10. Ground
11. Fire
12. Normal
13. Rock
14. Dark
15. Poison
16. Grass
17. Ice
First the primary type. We can eliminate the top 10 because the point of this CAP is neglected type.
11. Fire
12. Normal
13. Rock
14. Dark
15. Poison
16. Grass
17. Ice
Lets start from the bottom and go up:
Ice - Ice has two very bad problems:
1) Stealth Rock - This can be neutralized by a secondary Steel, Fighting, or Ground typing.
2) Bullet Punch - This can be neutralized by a secondary Steel, Water, Electric, or Fire typing,
As you can see the only type that neutralizes both weaknesses is Steel which leaves us with Steel / Ice. Steel / Ice however is 4x weak to Fire and Fighting and 2x weak to Ground which are all very common types making this a poor choIce for typing. This eliminates Ice from the pool of primary types.
Our list is now down to:
11. Fire
12. Normal
13. Rock
14. Dark
15. Poison
16. Grass
Grass - Grass is a mediocre typing all around. It is resisted by 7 types offensively and has 5 weaknesses defensively. It is outclassed by several other types and would have to have very good stats (Celebi) or a very good movepool (Breloom) to really be good for OU and I don’t think this poke will be getting Spore or having a BST of 600.
11. Fire
12. Normal
13. Rock
14. Dark
15. Poison
Poison – Poison is an interesting typing. It has a few bad weaknesses but several good resistances as well. The main problem to be circumvented with Poison is its weakness to the most common move in the game, Earthquake. This can be circumvented with Levitate, a Flying typing (both already suggested), a Grass typing, or a Bug typing. Flying and Levitate have already been suggested so I’ll ignore those. That leaves us with Grass or Bug type. Poison / Grass is actually a very decent typing leaving us weaknesses to Fire, Flying, and Psychic and resistances to Fighting, Grass, Water, and Electric. However the STABs of this typing are horrible and would most probably have to be a defensive pokemon. Poison / Bug is in a similar boat with decent resistances and nearly as bad STABs however it is stealth rock weak which is a very undesirable trait.
11. Fire
12. Normal
13. Rock
14. Dark
Dark – The Dark type is weak only to Fighting and Bug and resists Dark and Ghost with an immunity to Psychic. Dark can thus combine with almost any type to form a decent type combination.
11. Fire
12. Normal
13. Rock
Rock – Rock falls into the same boat as Ice since it is weak to Bullet Punch. A secondary Steel, Electric, or Fire typing gives the typing a 4x weakness to Ground, which is something that should probably be avoided. That leaves just a secondary Water typing. Rock / Waterl is weak to Electric, Fighting, Water, and Grass and resists Ice, Fire, Flying, Poison, and Normal. Offensively Rock and Water are very good STABs hitting everything in OU but Breloom and Empoleon for at least neutral damage.
11. Fire
12. Normal
Normal – Normal doesn’t leave very much room for creativity and is not a very interesting type and really has no chance of winning a typing poll, so I am going to ignore it.
11. Fire
That leaves Fire. Fire is weak to stealth rock and so to be viable without having overpowered stats and/or movepool it has to have a type neutralizing its SR weakness. Steel, Fighting, and Ground are the options for this. Steel / Fire and Fighting / Fire are both typings held by prominent pokemon in OU (Heatran and Infernape respectively) and so for creativities sake I will reject them. That leaves Fire / Ground. Fire / Ground resists Bug, Fire, Steel, Electric and Poison and is weak to Water and Ground. It also has extremely good offensive STABs (Look at Heatran’s two most common attacks).
So, our possible types gleaned from this analysis are as follows.
Poison / Grass
Weaknesses: Fire, Flying, Psychic
Resistances/Immunities: Fighting, Grass, Water, Electric
Rock / Water
Weaknesses: Electric, Grass, Ground
Resistances/Immunities: Fire, Ice, Flying, Poison, Normal
Fire / Ground
Weaknesses: Water, Ground
Resistances/Immunities: Bug, Fire, Steel, Electric, Poison
Poison / Grass seems to be the most unique typing of the three whereas Fire / Ground seems to be the best. However, since the goal of this pokemon is to have an underused typing I believe Poison / Grass seems to be the best typing for the job.
TL;DR I support Poison / Grass