It depends on what I have it on.
If it's on a 510 atty I'm dripping, I prefer delrin. I have a few delrin tips with larger bores than other tips so they're easy to drip through.
On an atty on my feeders or on just about any carto, I prefer ceramic. Both Avid and Empire sell the same black ceramic drip tip. It has a nice feel and resists heat very well.
I recently purchased a couple of blown glass 510 donut drip tips, and I could see myself getting to like such a thing, but unfortunately the ones I got don't actually fit my 510 stuff very well (or really at all). I'm looking forward to some trippy donuts being in stock soon.
I have some aluminum drip tips - I don't like them at all. I have some stainless steel tips that I also don't care for. And, I have some nickle plated copper tips that I do like but not on anything that will get them too hot. They work really well in conjunction with a delrin 306/510 drip tip adapter. It keeps them from getting hot and they have a big enough bore and smooth enough surface for dripping.