Try to think of everyone who might be willing to write you a positive letter of recommendation in your particular field. Generally, places requesting letters of recommendation will ask for at least two and no more than three, but it's a good idea to think of a larger list, in case any of your ideal candidates don't work out. Try to come up with at least 5-7 possibilities. How you choose to assign "strength" to the candidates will be relative, but in general you want to find a balance between the familiarity of the letter writer with you and your work, and that letter writer's standing in the field. While you might have had a better relationship with a co-worker or a TA for one of your classes, a letter from them may be less desirable than from a supervisor or a Professor who also knows your work, but may need some reminding. Consider collecting profiles or portfolios of your work to share with potential recommenders who may be less familiar with you and what you do. This is a good way to make contact with a successful person in your field and shows initiative. The easier you make their job, the more likely the candidate will be to do you the favor.

Summary:
Write out a complete list of possibilities. Order the list. Aim high.