You're a great parent when you did your best. We all have different circumstances and can only make the best of it. With that said the irony in this for me is that the best parents are the ones who aren't parents yet but prepare themselves to be the best parents when its time, makes sense? Getting a good education, financially sound, etc. Will go a long way so you aren't worrying over those issues after having children.
I feel that you are a successful pareng whdn your children come to you for advice and want to know how you got through this phase or that phase when raising them. It is also a good sign when your children make great decisions that can help them for the rest of their lives.
I believe the successful parenting is measured by the children. My parents were successful and I hope they taught me enough to be just as successful with my kids.