I would pick the male with the best physical condition or the nicest disposition. Another person might pick the male they like best or the male that will sire babies worth the most money. Choice is up to you. All the choices are equal as far as the genetics goes.
Expected result from yellow belly x spider:
1/4 normal
1/4 spider
1/4 yellow belly
1/4 yellow belly spider
Expected result from pastel x spider:
1/4 normal
1/4 spider
1/4 pastel
1/4 pastel spider (also known as bumblebee)
Expected result from mojave x spider:
1/4 normal
1/4 spider
1/4 mojave
1/4 mojave spider
Expected result from fire x spider:
1/4 normal
1/4 spider
1/4 fire
1/4 fire spider
The actual result may vary from the expected result.