Well, CDs (or DVDs, depending on how much you want to backup) are probably the most reliable medium you can use. If you burn a backup today, and take care of the disk, you can be pretty damn sure that the backup will still be good in 20 years.
But if you need high-speed, high-density backups, then tapes are your only choice.
DLT tapes are good because there is no physical contact between the tape and the head, so there (in theory) is no degradation of the tape when you do a read/write operation on it.
This is good if you are on a budget, since you will be buying less replacement tapes. But, if you aren't buying the tapes (you are working for a company or something) then the sky is the limit.
If you can afford the insane costs of the drive/tapes, a LT0-2 tape system will let you store 400GB on a tape.
I myself like Travan tapes (bit expensive, but nice) and the 4mm DDS-4 tapes are also great.