When I export as "even field first" or "odd field first", my video looks crisp and sharp but has those interlace lines whenever someone moves.. which looks terrible.

When I export "single field", I lose those ugly interlace lines, but also lose the crispness. All the diagonnals become jagged  and blocky....

Can anyone suggest how I might get a crisp picture and lose those interlace lines???

It depends entirely what your destination is.  For broadcast or DVDs for showing on TVs you shouldn't de-interlace.  It may look wrong on a non interlaced computer monitor but will be fine when shown on a screen that understands interlaced signals.   If you are making something for showing on non-interlaced displays e.g. computer monitors then de-interlace any interlaced footage in Avid using Fluid FIlm or some other plug in, and add graphics, captions etc on top of that before exporting using both fields.

regards

Brian