I know people who only use Hanna Andersson for their children's underwear. That's the only thing they buy from the catalog, and it's the only kind of undies their kids will wear.
Well, actually it's only one kid. Their kid had a hard time with underwear, and they found that the HA undies worked for her. That's what they always buy from the catalog. I think I was once looking (unsuccessfully) for discount codes for Hanna Andersson online, and came across a mainstream discussion about kids clothes, and some people there were saying the HA undies were the only ones their kids would keep on, too. So I got the idea that others had the same experience.
I've only had experience with the Hanna Andersson training underwear, and only a tad of experience with them so far; they're soft and seem comfy. Definitely not too tight on my lanky and slim toddler.
I've seen the Land's End ones previously mentioned. They have really good customer service and an unconditional guarantee, too. It might be worth it to give them a try first, and see if they meet with approval. Or try the OshKosh, if you've got a Burlington Coat Factory nearby. Because the Hanna Andersson clothing is more expensive. I find that the price is generally worth it for us (we like to buy the HA pjs on sale at the end of the season for use the next year) because they are comfy, they wear really well, and they fit for a long time. And many of the fabrics are Oeko-Tek certified, if not also organic cotton.