DES (5100 K/sec) has a 56-bit key; this is starting to become too small
for safety. It has been estimated that it would only cost $1,000,000 to
build a custom DES-cracking machine that could find a key in 3 hours. A
chosen-ciphertext attack using the technique of linear
cryptanalysis can break DES in pow(2, 43)
steps. However,
unless you're encrypting data that you want to be safe from major
governments, DES will be fine. DES3 (1830 K/sec) uses three DES
encryptions for greater security and a 112-bit or 168-bit key, but is
correspondingly slower.
There are no publicly known attacks against IDEA (3050 K/sec), and it's been around long enough to have been examined. There are no known attacks against ARC2 (2160 K/sec), ARC4 (8830 K/sec), Blowfish (9250 K/sec), CAST (2960 K/sec), or RC5 (2060 K/sec), but they're all relatively new algorithms and there hasn't been time for much analysis to be performed; use them for serious applications only after careful research.
AES, the Advanced Encryption Standard, was chosen by the US National Institute of Standards and Technology from among 6 competitors, and is probably your best choice. It runs at 7060 K/sec, so it's among the faster algorithms around.