If you want the ultimate replay viewer experience, try . It features: A functional seek bar (jump to any part of the song). Detailed hit diagrams.
While the standard client doesn't natively allow frame-by-frame scrubbing, using mods like Lazer (the next-gen osu! client) gives you a seek bar and variable playback speeds. The "osu!lazer" Difference osu replay viewer
Use OBS (Open Broadcaster Software) to record the replay viewer while it plays. If you want the ultimate replay viewer experience, try