Check out http://www.dorba.org for lists of all our trails around the Dallas Metroplex. I’m not sure if your skill level, but the most challenging trails (IMO) on that list that I personally have ridden is Northshore (west side) and Isle Du Bois (including loops D & E).
Northshore is our most popular trail by far, though.
Big Cedar used to be a very popular trail until land ownership issues came about, now it’s not tended to very well because the land owners are cumbersome.
I hear Boulder and Cedar Hill State Park are legit, but they are pretty far south for me and I haven’t gotten a chance to ride them yet.
Rowlett creek Preserve is a good ride with several fun loops. Oak cliff Nature Preserve is another good spot. One of the tougher trails (at least last I rode it), was Erwin Park in McKinney. Check DORBA or http://www.mountainbiketx.com for more in each area you are researching…it can be broken down by region. Lots of good info on both.
From where you’re coming from, these will be some of the most technically interesting, and fun trails. The above recommendations are all solid, but are much more XC/beginner oriented. So, if you’re only hitting a handful– these are a pretty solid mix:
DFW – Northshore, Big Cedar, Boulder and Sansom.
Austin – Reimer’s Ranch, Flat Rock Ranch, the Greenbelt, Bluff Creek Ranch
*Houston – Double Lake, Cypresswood Trails, Huntsville State Park, Memorial Park, Jack Brooks. (Keep in mind, Houston isn’t really somewhere people go for destination mtb– Austin and DFW have considerably more elevation changes to work with.)