Synths can do such a wide variety of sounds it is really hard to say that one preamp is "best" IMO. Also, generally, if not driven too hard, preamps tend (or should) be rather neutral.
What I noticed:
- api is great on anything, stays relatively neutral, and takes overload best with non-bass stuff
- neve-like (BAE & Nice DI here) take bass overload a bit better
- tube pre (sebatron with coloured mullard and more clear german tubes) is fantastic on TB303
; also it gives some "clarity" (maybe it is just the noise floor? ;-); too much overload = things fall apart in a bad way
- drum machines sound great through ANY of my preamps when slightly overloaded
If I could have only one it would be the api, because of the added sends & mixbus that can be driven (so functionality counts in here).
The most special is the tube pre and that one also made the biggest difference when using a JDI as compared to the preamp DI. So I think a DI per se is not always adding too much (it is great at killing hum of course but pure tone...in my experience it depends on the DI of the pre).
What i did notice is that furhter processing in the DAW seemed to sound really good after I started using pre's, but maybe that is placebo.
What sounds are you interested in to compare (besides a TB which I decided I want to do a nerdy comparison now anyways)?