My BF has been going to Fifth Avenue Alterations for years and swears by them. He's the picky one in the relationship, so when he took a look at a pair of jeans that I'd recently had altered elsewhere, he sweetly suggested that I take my next pair to Fifth Avenue where they would do a 'better' job.
The husband/wife owners are very sweet and good at what they do... I've been to see them now a handful of times and I've been pleased with everything I've had altered there (including jeans, tops, and even a dress). More importantly, my BF is pleased as well!
The only complaint I have is that sometimes it can take over a week so this may not be the best bet if you are in a hurry. Also, they're cash only, so be prepared!