Excellent question bound to stir up the pot a little...
For Journey $10 insert prefixes (as far as what we've seen for sale, asking prices, feedback from excellent sources, and other info, etc)
I'd put my money on the following:
1) 2002 K/D FEN
2) 2000 K/T FDU
3) 2003 J/D BEK*
4) 2004 J/D BER*
5) 2000 K/T FDZ
6) 2001 K/D FEC
7) 2002 K/D BEI
2000 K/T FDV
9) 2000 K/T FDT
10) 2002 K/D FED
* I think the "jury" is still out deliberating on these two inserts so its too early to say...they could very likely be contenders for the top spot with FEN and FDU.
AS for non-inserts (which are equally
or more important than inserts in this series)
1) 2003 BER
2) 2003 BEL
3) 2003 BEK*
4) 2000 FDY
5) 2001 FEE
6) 2001 FDZ
7) 2003 BEP**
2004 BET
9) 2004 BER**
10) 2002 FEN
* Again, its still a mystery exactly how scarce the 2003 BEK non-insert is and it may give BEL a run for the money. 2003 BEP and 2004 BER could well be contenders for 4th and 5th spot but I think there's been quite a few saved in UNC condition, which have kept their prices down. I suspect hoards of BET's and definitely FENs
NOTE: I just bumped up the BET over the BER (2004) as I feel (from what I've seen) that its definitely the rarer of the two