Dragon for secondary typing, hands down.
What do we get from it?
Half damage from: Fire/Water/Grass/Flying/Steel
Quarter damage from: Electric
Neutral Type coverage against every type and almost every pokemon.
Neutral Stealth Rock/Pursuit damage.
Downsides:
Double damage from: Ground/Ice/Dragon
Quadruple damage from: None
In CAP so far, we've covered all types but ELECTRIC, DRAGON, Flying, Psychic, Dark and Normal. We're using types that are neglected so far in CAP as well! It has great resistances though it's weaknesses are fairly common. As either an offensive or defensive pokemon it could work well, especially as Physical Offensive.
The most beneficial types I can see are Bug/Dragon/Fight. We've covered Bug and Fight already, let's go with a so far unused and popular type, Dragon. Bug has a SR weakness which could limit it's offensive and defensive abilities, and Fight, while effective against most steels, doesn't hit the most common pokemon Metagross for enough damage. Neither does Dragon, but that lends itself to possible Fire moves.
What do we get from it?
Half damage from: Fire/Water/Grass/Flying/Steel
Quarter damage from: Electric
Neutral Type coverage against every type and almost every pokemon.
Neutral Stealth Rock/Pursuit damage.
Downsides:
Double damage from: Ground/Ice/Dragon
Quadruple damage from: None
In CAP so far, we've covered all types but ELECTRIC, DRAGON, Flying, Psychic, Dark and Normal. We're using types that are neglected so far in CAP as well! It has great resistances though it's weaknesses are fairly common. As either an offensive or defensive pokemon it could work well, especially as Physical Offensive.
The most beneficial types I can see are Bug/Dragon/Fight. We've covered Bug and Fight already, let's go with a so far unused and popular type, Dragon. Bug has a SR weakness which could limit it's offensive and defensive abilities, and Fight, while effective against most steels, doesn't hit the most common pokemon Metagross for enough damage. Neither does Dragon, but that lends itself to possible Fire moves.