Minimonsta VSTi is good for synth bass. Basically it emulates that classic moog sound. V-Station & Bass-Station are also nice for synth bass. Ultra Analog VSTi is nice also. Sylenth1 is also very good, but the factory presets are more built for Trance type music, but with tweaking you can get some nice solid synth bass for that classic gangster shit. Surge VSTi is also good. Vanguard too, but it's a bit more for trance preset wise as well.
For synth bass, all those VSTs are great. They offer alot of presets to fuck with as well. Download them all. Trust me on this one you won't be disapointed. Their top notch vst synths.
For more acoustic/electric bass style of bass sound. I suggest more sampled based plugins. Plugins like Trilogy (haven't used yet, but it gets a lot of praise) & Hypersonic2 (I like it). There's also plugins that use like a combination of both samples and synthesis, like Morphine. I also like the freeware 4Front Bass Module vsti that sounds nice when played on lower octaves. There's also synths that use physical modeling synthesis that can sound highly real, like String Studio.
To bring some life or change the tone of some of your bass sounds you can also try Bass amp vsts. I really like Studio Devil Virtual Bass Amp. Compression as well.
Take a listen to this beat I just made a few days ago. I used Sylenth1 VSTi for the bass. No FX, nothing extra, other than than some slight compression & limiter on the master. Basically it's a deep moog style bass synth. Not sure what your after exactly, but I'm guessing something similiar to this shit. Pretty much anything is possible with those synths I mentioned. From deep low bass, buzzy, watery squelchy,etc... You just gotta fuck around and tweak shit, but the presets are pretty good to start with. Sometimes it can be as easy as playing something on a lil higher octave or lower (try it all when testing the presets), or lower/higher the cutoff/res on the filter. Gotta choose the right kick too, to go along with the bass. If their fighting for frequency, they might phase out or distort.
http://www.zshare.net/audio/5354447369003aab/