All things considered, either water or (maybe) electric.
Water
- only 2 weaknesses, grass & electric
- electric is covered by water/ ground types like Gastrodon or Volt Absorb Lanturn
- grass liability is covered by ice beam (which nearly all water Pokemon can learn)
- you can already have 1 water type as a starter, Oshawott
- Samurott can learn Waterfall, Dive and Surf, Swanna can learn Fly, Surf and Dive and Bibarel can learn all HMs except Fly
- a few water types can learn Flash as well, e.g. Lanturn
- water is the most common type, meaning you have the most Pokemon to choose from
~ downside is that you will struggle at Arti and Kamitsure's gyms
= you can trade for a Volt Absorb Lanturn before Kamitsure, and get a dual water/ flying type before Arit's gym
Electric
- only 1 weakness, ground
- ground negated either by Rotom, Emolga or Eelektross
- ground is neutralised by Galvantula
- Rotom forms means you will be able to switch its type according to what your team needs
~ downside is you'd be hard pressed for HM slaves; Zekrom is the easiest available Electric type that learns Fly in B2; the other electric types that can learn Fly are Zapdos and Thundurus
~ best electric HM slave will be Lanturn; it can learn Surf, Waterfall and Dive
~ there's also not a lot of Electric Pokemon to choose from
Ultimately, I would advise you do a mono-water game.
