For learning:
GTP_Timppaq[FI], GTRP_RudiRaser[DE], GTP_mad94d[AU]
as I can usually adapt what Timo, Rudi and Madison do to make myself a little quicker here and there.
For entertainment and awe:
The above plus: FAK_EV1L[IT], zabeu[GB], GTro_Ramon[ES]
Their styles don't really match mine but their replays are just poetry.
For the GT5 demo I used Timppaq's normal car ghost to great effect and FAK_EV1L's tuned car ghost the most. As soon as I stopped using the ghosts I improved much more, but needed their replays and ghosts to get the basics.
All the best
Maz