Ice/Fighting is Super Effective on 347 Pokemon.
However, since no Pokemon has this type combo, Ice/Ground (the Mamoswine line) would have the most Super Effective coverage with 342 Pokemon hit for Super Effective damage.
There are some type combos with no or very few Pokemon resisting.
Dragon/Fire (only resisted by Heatran w/ Flash Fire)
Dragon/Ground (only resisted by Skarmory, although Shedinja is immune to this)
Fighting/Dark (only resisted by Heracross, Croagunk, and Toxicroak)
Fighting/Ghost (nothing resists this)
Ice/Electric (only resisted by Magnemite, Magneton, Magnezone, Rotom-H, and Rotom-F)
Fighting/Rock (Hits 338 Pokemon for SE damage and is only resisted by 11 Pokemon)
Rock/Ground (only resisted by Breloom, Torterra, and Virizion)
Water/Normal (Only resisted by Empoleon, Dialga, Giratina, Frillish, Jellicent, Ferroseed, and Ferrothorn)
Fighting/Flying (only resisted by five Pokemon)
Ground/Flying (only resisted by eight Pokemon)