if i has a dime for every time this question gets asked.
1. Do a search
2. go to Invalid Link Removed to hear a bunch of different preamps
3. go try a bunch with your bass, your amp or poweramp, and your cabinet(s)
It depends upon your budget too. Cheaper good preamps incluse both the solid state and tube BBE B-max preamps, and the sans amp RBI. Good discontinues ones to look out for used are the swr grand prix, demeter "blackface' 1 and 1/2 space VTBP-201, and trace elliot v-type. More expensive ones available both new and used include the alembic f-1x, demeter 201s and HBP1 pre's, aguilar 680 and 659 pres, kern, read purity, eden navigator, and a bunch of others.
FWIW, i would decide upon how much your budget is (you can spend anywhere from $150-$2000+). then check out preamps based upon their flat tone more than all the bells and whistles available on them. The consistent favorites of most players are very simple preamps with few controls and mondo tone.