I would guess by the lack of responses no one is taking your question seriously for many reasons, so I thought I'd at least respond with a species. Although anyone responding to your question is forced to make several assumptions to even begin to discuss the matter with you.
For example what country do you live in (if in the US what state, as some have their own laws on insects too)? How much experience do you have with praying mantises, and insects in general? How much money do you want to spend (some of the more rare/advanced species can cost over $70 each)? Can you provide proper feeder insects year round, temperatures, and humidity levels? etc...
There are 2300 to 2400 different mantid species in the world, with only a tiny percentage of that kept as pets or available from mantid keepers and/or online stores. Even then it varies depending on what species they currently have available.
Megamantis and/or giant shield mantis is two larger ones I've been after for awhile now. Idolomantis diabolica get the same sizes as the Chinese, so I'd consider them a contender as well.. but their care is recommended for experienced keepers.
Rhombodera megaera and Hierodula majuscula rival most in culture currently. Cilnia humeralis is probably the bulkiest but isn't really in culture right now.
It was not for sale, they just had a couple of adult females from different species to show (loved the D. Lobata) apart from nymphs they were selling, but when there are babies, I'll probably get them