Well, 18 megapixels to start. Its got the polycarbonate/magnesium alloy build that ups both the Pentax and Sony, the manual interface I prefer over Nikons, its got the brighter pentaprism with 100% view, and the fastest AF of any listed.

Though I'd pick full frame with A850 over any of those. For weddings/receptions/bday parties you would have a serious advantage with full frame, to really make good use of the 15-30mm focal range. The outdoor specific features aren't neccesarily a major advantage for full frame, but it is still nice. Of course, spend a little more still and the A900/5D/D700 also become options.

All ultimately falls to how much you are wanting to spend, your question is very vague as to budget, and different options are better at different budgets. Common rule for starting an investment in gear is to use a good chunk of the budget on quality optics, rather then spending all on the camera body. If your budget for example is $2k, then its good to spend some $700-$1k on Camera body, and the rest on good glass. Optics are usually something that outlasts several camera body upgrades.