I pulled this off another site. Apparently, i386 is considered the safest. While others may work with your system architecture, the i386 is guaranteed to work.

i386 - Optimized for 386 and above processors (386, 486, 586 (Pentium), beyond).

i486 - You rarely see these if ever. Optimized for 486 and above processors (486, 586(Pentium), beyond).

i586 - Optimized for 586/Pentium processors (586 (pentium), beyond).

i686 - Optimized for that beyond category (Pentium Pro???, Pentium II, Pentium III, Penmtium IV, K6-x, Atholon, Duron, Celeron (later only???), Thunderbirds, etc).

src - Source package. These are rebuilt into packages and can optimize for whatever platform you have. They take the longest to install.

References:
linuxiso.org
resellerratings.com