If you are allowed to use it, TI89 Ti all the way..
you will need to take some time to learn to utilize the interface, BUT with calculus it is a godsend to save you from making stupid mistakes.
the other big calc that I upgraded from was a TI-83+, easier interface, but nowhere near the ability. I broke down and spend the $150 on the 89 when I took Diff-Eq.
as far as non-graphing calcs.. the TI30xs http://education.ti.com/educationportal/sites/US/productDetail/us_ti30xs_mv.html has been great. I use it alongside the other calc frequently if i dont need the power of the bigger one. The mathprint mode and fraction input makes it the easiest interface that I have encountered (and it has better button feel then many others in the TI non-graphing line) so I can input faster into it and with less mistakes then the graphing calcs.

