New Exhaust

The stock mufflers on my 2005 MG TF Spark SE are worn out and in need of replacing. What brand of exhaust kit can you suggest?

It depends. Do you want standard or sports? The standard Mild steel exhausts are quiet and less expensive or most sports exhausts are stainless, more expensive and give a throatier noise.
Cost saving, any pattern replacement from the likes of ECP will do.
If you want stainless and a throatier sound the cheapest is Toyosport.
I prefer the sound of the Satur Daytona, but other makes have their fans.

MGOC spares do standard and sports exhausts in mild steel and stainless. i've not fitted one to a TF, but fitted to other MGs in the past and the quality has been good. Stainless would appear to be the way to go if the car is a keeper - just sold a 10 year old Skoda with 100k on the clock, exhaust was original and almost like new.