what brand of transmission oil should I use to flush my 2004 Impreza 2.5rs tranny

Don't get fussy. The important issue is to get the trans clean. Start by draining (DO NOT "POWER-FLUSH!!) the old ATF, replacing it with exactly 3.5-3.75 qts good old Dexron ATF. Drive for a few hours, then check the dipstick: if still very brown or dark gray DO IT AGAIN! The 4EAT holds 8.8qts, so you're only replacing 40% with each (gravity) drain and refill, so on especially dirty ones I'll do it twice, then again annually. The differential oil should be checked to see if it needs maybe a pint of gear oil...do not confuse the two! The REAR diff oil is good for life. (Again, powerflushing the 4EAT can easily result in detritus being back-flushed into the sensitive valve body, sometimes destroying all function, so don't fall for one of those $89 "automatic flushes" that pump-owners need to amortize the cost of. Trust gravity and repetition to obtain cleanliness. Ignore the filter...they last forever.)