budget and style?
For vocals, any of the neve style pres are usually popular. I use a GAP pre-73 for vocals most of the time. I also have an API clone I use a lot too though. For vocals, the only pres I really don't like a lot are the super clean ones (i.e. the ones that come in most mixers/interfaces and RNP) even though they are great for everything else.
But, even the built in pres in mixers/interfaces will get good results, and if you're not getting good results with those, the pre won't be the magic bullet. If you're ready to step it up a bit, the neve style are what a lot of people like and for $300 you'll find a ton of uses for the GAP.