The XP30 is a fantastic bet for heaps of great sounds and good expansion.
A Triton LE is also excellent but it has the Korg sound as opposed to the Roland sound. It all depends what sound you like better- I like both!!!!!
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Another couple of options are Alesis QS series. I owned an S4plus module and really liked the sounds of it. The Yamaha S30 is also a good bet and also check out a 2nd hand EX5. These are great synths. For something new the Motif is great!
It all depends on how much money you have and what 'sound' you like. No manufacturer makes 'bad' sounding synths anymore!!!! ]
Emu systems are about to release some new keyboards well worth checking out if you like a more 'American' sound. They are based on the synth engine of the Proteus 2000.
So there are heaps of options for good synth sounds
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