For a lot of people Propellerheads Rebirth 2.0 features the best 303 emulation.
(It’s also a Roland 808 and a TR909 emulation, 2 other great vintage machines, dedicated to drum synthesizing). It’s a standalone software, so not a VSTi, but you can use rewire to link Rebirth to Cubase.
And the best thing is…as it has been discontinued by Propellerheads, it’s now avalaible at
rebirthmuseum for free! And with a lot of mods (new graphic interfaces and innovative sample sets) and addons like Pattern Master (to program Rebirth using a simpler piano roll view) and ReNovator for creating your own mood. The sound quality is still excellent, so it well worths the download, which is more than 500Mb, as it’s include all the RB-338 CD-Rom files.
It’s available for free for a long time now already… but it’s still a great tool indeed 
Now I just wish I could use it within Renoise 