I started with Allen's and really like it just fine. No rashes or build up, but didn't do much for the stains.
I tried Tide since I had a friend that swore by it and dd got a TERRIBLE detergent burn. I felt like such a bad mommy. It definitely brightened up my inserts though!
I tried Sun when we went on vacation and I forgot my Allen's. It worked fine with no rashes or build up. I felt like my diapers were not as soft though and it did nothing for stains.