N. peltata, N. sibuyanensis, and N. burkei all have bulbous lower pitchers. Peltata is pretty adaptable to a wide range of temperatures (found from lowland to highland elevations in the wild), but tends to be on the pricey side, since it's relatively new. Sibuyanensis and burkei like higher humidity and a temperature drop at night, but are cheaper than peltata at least. Then of course, there's the obligatory N. ventricosa, very well known for its ease of growth and squat pitchers. Hybrids involving N. glabrata tend to have cute and bulbous pitchers, but are also somewhat picky.
N. attenboroughii also has very nice bell-shaped lower pitchers, but even if you do happen to get hold of one somehow, the thing is rather annoying to cultivate and grows slower than a bristlecone pine. I think the best candidate for a windowsill grower would be N. ventricosa, but if you want extremely bulbous pitchers, you'd need to go for something else. Good luck!