Bali seems to be a favorite amongst many, many K users. The regular powdered stuff is cheap, but people tend to prefer the "aroma" of a good bali blend.
It really depends with you are looking to get out of your Kratom as far as what you should buy. There is stuff called UEI (ultra enhanced indo) that is killer, killer stuff. If you don't have a kratom tolerance, the smallest little bit works wonders. Helps with pain management (if you're into that sort of thing), helps you sleep, and gives you a wonderful, almost opie like buzz.
Bali can actually have some of those same effects, but I think they are much stronger with UEI. For me, bali tends to give me more energy, and less of a euphoric, sense of well being type of buzz that UEI gives you. But it's still great. Cheap, awesome stuff. Some people blend the 2. Burn a bit of Bali and toss in the smallest bit of UEI. Actually some vendors sell it premixed. PM me and I can show you a kick ass vendor who I buy through. Great, great, great.
It looks really expensive, as it costs more then a99% of other kratom blends, but IMO can be very worth it. However, it can be VERY habit forming if you get into taking too much. WD's cam be a real bitch, so treat Kratom with the respect you'd treat any other drug. It's legal and all, which I think throws some people off, but a quick google search would show you that people have gone through some very real and unpleasent WD's when dealing with this stuff.
Anywho, that's my Kratom 2 cents. Do with it what you will...