I have used G.O.T, Cyanogenmod 6.1 and Cronos before.
The best is Cronos in terms of battery life.
I am using Cronos ver.1.7.0 at the moment but as of now, Cronos just released ver.1.8.1 (based on the leaked Brazilian kernel for Milestone).
What I like about Cronos (ver.1.7.0) is that it is clocked at 700mhz and undervolt to 49vsel and yet it feels as fast as when I had overclocked my stock Android 2.1 to 900mhz 56vsel.
The problem with G.O.T is Facebook sync doesn't work and at default speed of 550mhz, it feels laggy.
The problem with Cyanogenmod is it eats up my battery life crazy. This is how I conduct my test:
Back when I was on stock Motorola ROM Android 2.1, I usually browse my usual few websites on my 30 minutes trip to work on the public transport. And during that time, after 20 minutes of browsing my favourite websites, my battery level drops by 10 percent.
When I was on Cyanogenmod 6.1, that same 20 minutes of usage took 30% off my battery life.
So far Cronos (ver.1.7.0) gave me similar battery life (if not slightly better) to when i was on stock Motorola ROM Android 2.1
Here's the link to where u can located the necessary files from Cronos:
Cronos Android, Android on Steroids! • View forum - Official Releases
Note to newbies:
The big problem with the stock Android 2.1 rom of our Motorola Milestone is the locked bootloader. So the most straight forward method to get you starting on your journey to installing custom rom is flashing the G.O.T sbf file as described in this guide:
[How To] Install The New GOT Update On Milestone - xda-developers
Once you have successfully installed G.O.T Froyo ROM on your Milestone, remember to reboot into Open Recovery and Root your phone first and then do a Nandroid backup.
Nandroid backup is the failsafe for us. If anything goes wrong, we can always boot into Recovery and restore a Nandroid backup.
Once you get bored with G.O.T Froyo Rom, you can start to read up on how to install the other various custom ROM for Motorola Milestone which requires a different Open Recovery such as Androidaini Open Recovery to install their custom ROMS.