American 2-row is the cleanest base malt choice that will allow the hops to pop in a West Coast American IPA.
Pilsener malt is also quite clean, but different tasting, and more associated with Lagers.
If you want to have a little more malt taste, you could go with English 2-row, but not the heavier selections like 100% Maris Otter. You could even mix American and English 2-row for a hybrid base. If this is the case, you could use about a 1/3 of Maris Otter or Golden Promise, with the rest being American 2-row.
For extract brewers, go with Extra Light DME or Pilsen DME... either Briess or Muntons. You could even add <10% Wheat DME for some interesting complexity.
When brewing, be sure to add 1/2 or 2/3 of your DME at flameout (instead of all of it at boil start) with 10-13% corn sugar for further attenuation to combat the sluggish fermentability of the extract and be more West Coast.
Light Crystal (10-40L) or Honey malt can be used in small percentages (<6%) for added toasty caramel and/or honey notes.