What do you mean by "sound bank"?
External modules and synths are out. Software is the best solution. For realistic instruments, the key is the size of the instruments (in terms of memory size). And because hardware are limited in this aspect, realism will always lack behind software.
Anyhow, if you talk about hardware, the most realistic instruments you can find are in Roland. Provided "realistic" meants emulation of real instruments (ie strings, woodwinds, piano, guitar etc). Electronic sounds belongs to another category - you can't call them "realistic".
For software, it comes in 2 forms: 1. samplers (instruments being sampled note by note and in different articulations and playing styles), 2. Synth modelling (when very small amount of the instrument is recorded and using modeling technology to make it sound real). Realistic samples are huge in size; modeling instruments are much smaller in memory but require larger CPU processing.
Needless to say, samples are the most realistic. but their problem is the difficulty to play in real-time. Most require sequencing to get it realistic (because of the different playing styles and articulations). And they come in various forms: 1. VST plug in instruments (these are the most common nowadays), 2. Native sampler formats (require a softsampler to run it: like Kontakt, Gigastudio, Halion, EXS24 etc).
Modeling synths has potential. Although they are not as realistic pure samples, their strength is in the playability. You can play them in real-time and the software recognises how you play and changes the instrument's playing style. An example is Wallendar brass instruments (
http://www.wallanderinstruments.com/) which had been having a lot of attention lately (and I can't wait for them to come up with a saxophone). This is the most realistic out there - in some cases, even better than samples.
There's also samples that uses modeling synths (a hybrid). They are usually still regarded as samples, but they incorporate modeling technology to get them as realistic as possible. And of course, Garritan's Stradivari. They uses their so-called morphing technology to make their samples both realistic and playable live.
So, when you say sound bank, what do you mean? Do you mean the midi bank from your soundcard? If so, you're stuck - because they are the worst. If your soundcard supports soundfonts, you can still find some decent soundfonts to load into your soundcard. But they are far from realism compared to samples. Soundcards usually don't let you load soundbanks into them - except for some like Emu/Creative which you can use soundfonts.