First off - right now one of the most important things to consider is the sys version of 6.5 you want to use. Most of the ones that start with 23xxx are pretty stable. Ones that start with 219xx are typically experimental. Recently, someone got there hands on one from a Topaz (2182
that is supposed to be very stable - but it's a little older than the 23xxx ones.
Second - you need to decide what functionality you want to include. Here is where I think you have two options to start your flashing oddessy.
1) Go with something super stripped down first. Then use cabs to install what you find you are missing or want. Write down everything you find you really like and/or need. Since you are on Sprint I'd suggest
JD's ROM - very clean, always a good choice.
2) Go with something loaded. It'll likely have everything you need. Just go to the updgrades sub forum and pick one - Mighty Mike & Juicy are both very popular, and thus have a pretty large user base, but there are a ton of others that have good stuff.
You can also check out the links in my sig - you might find something there you like as well.