Best options :
1. BEST (IMO) - Go see live shows, and buy the same gear the bassist has if you like his/her sound. If they can do it so can you.
2. NEXT BEST: Try them out for yourself (this is the golden talkbass advice), BUT in my humble opinion, amps can sound very different on a shop floor than they might sound in a full band setting.
3. NEXT NEXT BEST: Go with a trusted brand, popular model (plenty of people here love Fender, Hartke, Ampeg, GK enough that you know it can't be all that bad)
Anecdotal advice: I got the Hartke LH500 and it is a great simple amp ( i got it used for around $200). A good versatile foundation for a rig which can be customized with pedals and cabinets to your liking.
Overall, pretty good advice but I would caution that option 1 applies only to live shows in small venues where you are actually hearing the bassists stage gear (as opposed to the P.A.). I'd also recommend chatting to the bassist between sets. Most are only too happy to chat about gear with a fellow player (I know I am) and you can learn a lot from these guys/gals.
As for worst amp.... so many to choose from.... I'll discount the ones that were clearly underpowered and say the SWR SM400. For a supposedly "pro" amp, that thing was lame.