Depends on exactly what you're looking for. If you like intense g-spot stimulation, maybe a nice glass dildo with a curve to it like Bent Graduate
or a little bit less of a curve like Midnight Twist.
Glass is extremely safe, can last a lifetime, can be sanitized, and you don't need a ton of extra lube because it's naturally slick. However, there's no give or squish to it, and if you go a little crazy you could bruise your cervix (in theory, I've NEVER had that problem but some people do talk about having done it).
If you're looking for something with a little more squish to it, try out a quality silicone dildo. Some are extremely natural feeling, like the O2 and VixSkin lines, which are made of a core of very firm silicone, and have an outer layer of very squishy silicone. One of my most favorite VixSkin toys is Mustang.
Again, silicone is extremely safe, can last a lifetime and can be sanitized.
If you're used to a jelly toy, the dual density silicones will probably feel most similar to you. You can do a search on the both O2 and VixSkin here on the site to see all dildos in that particular line. If you're looking to branch out to other safe materials, you could try looking at glass, stainless steel or ceramic. Hope this helps some!