I did a lot of research when mine were on formula, and I think the best one on the market is Baby's Only Organic Toddler Formula. Don't be discouraged by "Toddler" in the name - I called the company to inquire and they said it is named that because the company's philosophy is that children should be breast fed until one year, but it meets all of the requirements of baby formula. I brought the fact sheet with all of the ingredients/nutrients into my ped's office and he said it was the best one he'd seen! I've only found it locally at Whole Foods, but you can also find it online at diapers.com. On their website, they have a comparison chart to what's in their formula compared to other brands (what I showed the ped.) http://www.naturesone.com
I have triplets, now 15-months-old, who were preemies, so we had to use Similac's Neosure for a while, but when we got the go-ahead I switched. I never breastfed - pumped for a week when they were first born (they were too little to feed from breast or bottle), but it didn't really work for me. As someone else said, do not let anyone make you feel guilty about your decision to go to formula - formula's are wonderful now, and a happy mama is an important key to a happy baby! Mine are all thankfully very healthy, thriving and did very well on formula. I would not use anything but organic dairy - it is appalling what is done to cows and to the milk - all the gross hormones and antibiotics they put into non-organic formula - not to mention the genetic-engineering....wonder why girls are hitting puberty earlier & earlier? But I'll step off my soap-box now!
I think Similac is a decent brand, tried their organic formula for a while. I had received a free case of Enfamil, but one of my son's had a horrible reaction to it - couldn't keep it down. I can only speak from my experience, but switching formula's never had an impact on my children - I think it's important to find one that agrees with your baby - but if you do plan to switch around, give each a couple of weeks.
Good luck!