One of the most popular and better aftermarket ROMs for the Galaxy S2 right now in my opinion is the Batista ROM for the Galaxy S2, it comes with minor tweaks to the UI (although still pretty much Samsung TouchWiz UI), 1.6Ghz overclockable kernel, and other goodies. I think this is the best Galaxy S2 ROM out there right now.
Tagged: galaxy s2
Once you’ve made a backup of your current ROM, you will also want to learn how to backup and restore with Titanium Backup app as this method will allow you to quickly restore your apps when you install a new ROM on your Galaxy S2. (The ClockworkMod method backups up your WHOLE system including the ROM itself so you need another method to restore your apps such as this Titanium Backup app when installing new ROMs.)