I'd like to second the Dragon/Steel typing that Deck Knight proposed. It resists the main spamming types of rain teams, Water and Electric, without benefiting from rain by having STABs boosted or weaknesses eliminated.
How it deals with Dragons is mostly up to its stats but at least it isn't weak to Dragon and can fire back its own Dragon STAB super effectively. Opposing Dragon-types would have to resort to Fighting and Ground coverage moves to hit super-effectively, which are both very common immunities in OU.
As for sun teams, this typing is not outright weak to them but unless it has epic bulk it won't be able to stand up to powerful Sun-boosted fire stabs from the likes of Volcorona. Serving on sun-teams, it does add another ground weakness common to Sun-team pokemon, but it adds a Water resist and a SR resist, which are both very valuable to a Sun-team.
For these reasons I see Dragon/Steel as the best type for this concept, seconded by Water/Steel and Dragon/Grass both for similar reasons.