The DS version is the best, mechanically superior and the most up-to-date.
The PS2 version is fine, and has cool 3D graphics and a lovely orchestral soundtrack, but has less features overall and is only an fan translation which some people might prefer (I don't know how accurate it is, but it was serviceable enough, if a bit rigid), but I personally love the silly accents in the official releases.
The SNES version is also fine, and is the "purest" if you like that. I don't like that half-tile movement that all the SNES DQ games have so I found it really cumbersome. The graphics are really basic compared to all other SNES DQs too.
Though with DQ5, any version you pick is still great because it's such an excellent game.