A couple more of the older samplers with very distinct sounds:
Ensoniq Mirage (8bit, 32khz max with analog filters)
http://www.vintagesynth.com/ensoniq/ens_mirage.php
Yamaha tx16w (12bit, 50khz in mono with digital filters)
http://www.vintagesynth.com/yamaha/tx16w.php
I still keep the tx16w around as it has 8 outs and i have this "pie in the sky" idea of setting it up as the brain for an electronic drum kit later in life. Or i'll pair it with the Atari ST for a retro project.
These were the two I learned on and can honestly say that you will learn things using PITA devices that you can not learn elsewhere.
Sometimes easier isn't better.
A newer sampler with a very distinct sound is the Ensoniq ASR-x (black.) Not so much for the converters but for sonic capabilities afforded by the way they implemented the envelopes, routing and effects.
Understanding how to use a stand alone sampler without the whistles and bells of modern gear can open a whole world of possibilities that may get over looked in this point-click instant gratification reality.
You get real critical of the samples you are willing to take when you only have 720kb of storage to work with
Though tt is definitely not for everyone.