Well sadly that is kinda like asking what cola you prefer. Coke, Pepsi, RC.
Frame wise do you like the OE look of the DSE, or the fabed look of the others? Do you want GM control arms or custom? Why do you want an aftermarket frame? I guess we need to know what your thoughts, plans, and budget is first before we can help.
With that said, DSE, AME, 21st, ST, WD (if you can find a used one) all make nice stuff! Or if you want to upgrade the stock frame DSE and or ATS have kits for that too.
Have you done a search in this forum? There has been lots of talk on this exact topic. One things for sure, we love to help people spend there money!!
Oh, and Mario thanks for joining the site and asking for help!