There are no "best" items..
Troll : If you got many stunners/disablers in enemy team . get first powertreads/phase boots, then BKB which is very important, after you can go for lothars or dagger so that you can reach the enemy and perma stun. after you can go for butterfly
Rilay : phaseboots,dagger (then you can go for support items like euls, shivas or guinsoos or mekka or you go for a damage build with aganims and bkb)