I am a great fan of to-do apps and services. I have indeed tried many of them in the last few years. The core issue here, is that are many target users, each one with different needs. The universal ideal solution doesn't exist because the more features a service offers, the less minimalist and time efficient it risks to be.
Wunderlist is a relatively new option you might want to consider. It's developed by the young and dinamic german start-up 6 Wunderkinder, founded in August 2010, who is working on a new online working platform for the future.
I believe Wunderlist is a very time-efficient solution for the needs of busy medical students and researchers.
It's free. Business-oriented multiplatform to-do services can be very pricey.
It's multiplatform. It syncs really smoothly. Android app coming soon!
It has sharing features: via email, via the app itself, via Cloud App. Ideal for study groups. I wish there was a blog widget too.
Tag-context features are missing: if you really need more complexity I suggest you consider buying Appigo's TODO.
I hope this was a useful tip. Here is a minipoll on how you deal with tasks: