I personally like what Samsung has done with their Samsung TouchWiz UI on the Galaxy S2 but if you don’t like it and you want to be as plain Android as possible, you can try the CyanogenMod 7.1 for the AT&T Galaxy S2.
This is a great ROM that strips pretty much everything and for some of you, this may be the perfect ROM.
CyanogenMod 7.1 is still experimental but you everything is working out of the box with up to 1.6Ghz overclocking. Try it out and let me know what YOU THINK.
Download ROM and GoogleApps here: (Note: You will need to flash the ROM then the Googleapps right after in CWM.)
Download CM7.1 and GoogleApps