So I know a little more about the southern metro than I do for northern Harris County and Montgomery county.
For Montgomery county, you have Montgomery County Hospital District EMS. A good repuation for a service. About an hour north of Houston, but they are getting at least one hospital certified as a Lvl 2 trauma center up there.
In Houston you have TMC (Texas Medical Center). It's a HUGE multi-center area. 2x lvl 1 adult hospitals (Memorial Hermann and Ben Taub) and 1 pedi lvl 1 (Texas Childrens). City of Houston is run by HFD.
South of there you have the City of South Houston (third service EMS), Clear Lake Emergency MEdical Corps (non-profit private that does some 911 and transfers in Harris County), Acadian (private, does some 911 and LOTS of transfers). Plus all the associated small volly depts and hybrid depts (Friendswood Fire/EMS, Southeast Fire, etc)
To the east of there you have City of Alvin EMS (NO!), Angleton Area Emergency Medical Corps (Meh).
Farther south you have Galveston County. League City is a city third service EMS. Galveston County Health District - Galveston Area Ambulance Authority covers parts of Galveston County (Bacliff and Hitchcock), as well as all 911 on Galveston Island itself (I work on the island)
You also have La Porte EMS (third service city), Lake Jackson EMS (great dept, to the west of me in Galveston), Texas City Fire, (transporting career dept).
I'm missing services but you get the gist. Lots of services.
As for hospitals, other than the Houston hospitals, you have Clear Lake Regional Medical Center (trying for lvl 2), Bay Area Regional Medical center, and Houston Methodist St Johns, all in the Webster Area.
Galveston county have University of TX Medical Branch - Galveston (a Lvl 1 Trauma center), on the island. And UTMB is opening another hospital in League City.
Any specific questions, let me know I might be able to help.