There is the MG one which is the most common. It started life way back when the F was first introduced. It has had different seals fitted over the years and has a light coloured fabric headcloth.
Then there is the Heritage Hardtop which was the up market one offered as an optional extra not long after. You can tell this one by the two rain channels along either side and the MG badges on the seal where it meets the rear of the side window. They also come with a leather headcloth.
Next is the racing hardtop which is very basic and has the lip above the rear window. Has holes in the back window and doesn't have any headcloth.
All of the above are made of GRP and when originally introduced came in either black or white. Coloured oned to match the car was introduced at the time of the MY2000 MGF.
There have also been one or two others over the years made by people like Mike Satur (aluminium one) and Krafthause and Smith and Deacon. There was even one with a sunroof fitted.
Here's a picture of my Heritage top (you can only see the badges) -
and one with the sunroof -
Colour coded MG hardtop -