Poison / Dragon
Electric / Poison
Poison is definitely the way to go for a defensive typing here. Resistance to Fighting and Bug especially make it able to switch into common battle situations, while the common Ground weakness is one of the most easily exploitable in the game. Toxic immunity and Toxic Spikes absorbing are also top-notch advantages.
When coupling with Poison, I think we'll get the most mileage out of the Dragon typing. Defensively, the Dragon typing adds some amazing resistances, notably to the ever-present weather-based Water and Fire-type attacks. It's Dragon and Ice-type weaknesses are common, but they are never seen together and are only in a handful of instances combined with Ground or Psychic attacks, so Disabling incoming attacks may be advantageous, since the typing offers a good amount of resistances elsewhere. The typing is also helped out a lot defensively by common Steel-type teammates, notably Bronzong and Skarmory, who won't have the burden of sharing Sketchmon's Ground weakness (and Poison/Dragon covers their weaknesses right back!), making the CAP easier to fit into familiar teams and strategies. Offensively, Dragon is a brilliantly neutral offensive STAB. Being (presumably) walled by the ubiquitous Steel-types in OU, offensive Sketchmon is (presumably) forced to choose between either a boosting move or a coverage move necessary to power through the most common defensive type in the game (or choose the underwhelming Hidden Power, if it is learned naturally). This would make Sketchmon have to play nice with teammates in order to pull off a sweep, opening more doors for creative uses of Sketch as outright sweeping becomes too much of a hassle or if Magnezone support just isn't worth it (again presumably).
That's not to say that Electric isn't also an amazing type to pair with Poison. Opponents will be gunning for that Ground weakness (again, easily exploitable), which is compounded negligibly by its only other weakness to Psychic. Defensively, it's just a little less desirable than Dragon in terms of useful resistances, but it still has an edge when it comes to weaknesses. That one's a give and take, imo, and made it hard for me to decide between the two. Offensively, Electric attacks do suffer from immune opponents, and unfortunately those opponents Ground-typed, but at the same time it gets better SE coverage than Dragon and remains neutral in general. Electric also has a good deal of fun STAB options for use with the Sketch slot, much more so than Dragon, whose strongest attacks have pretty severe drawbacks. Another excellent option.