Hard is somewhat subjective, but if you mean steep then Chris has a good list. I would add the Daniel Webster Scout trail to that (up to Madison). I've heard the Desolation Trail is a nice steep ascent to Carrigan, although I haven't had the pleasure to do it yet. I just did the Walden trail up to Passaconaway, which had steep some steep stretches on the way up from Paugus Pass. The Zeacliff Trail up to Zealand is also a less traveled steep trail.
The Hi-Cannon trail is a pretty direct ascent on Canon, and the Wildcat Ridge Trail up from 16 is quite steep and rugged, but both are highly traveled.