1. MTA:SA - The best MP mod for San Andreas from a technical standpoint. Great scripting interface and very well documented. If you can think of it, it can be created in MTA:SA. Furthermore the sync is fantastic, even compared to other games. Great community too. This is where I spend most of my gaming-time, and have for the last 10 years or so.
2. LU - Lots of potential, yet I feel like people have barely scratched the surface. Or at least at it stands today with only a handful of servers. So there is room to create something unique and fun. I'd love to put some more work into creating scripts for LU.
I have played SA:MP before MTA:SA was released, and occasionally after that. Not sure how it stands today, but it doesn't seem to have changed much since then. I like MTA better looking at it from a technical standpoint, but I think SA:MP still has a bigger community.
Have played the others too, but too long ago/not enough experience to compare.