Spring RTS Engine

I would shamelessly plug http://www.beyondallreason.info/ for this.

which Spring RTS based game is mostly like Supreme Commander Forged Alliance or wants to be like it?

What do you mean by this? You mean pvp-oriented combat RTS with humanoid robots, tanks,etc. where people start with their commander?

In that case I'll shamelessly plug the one I develop, Metal Factions

There's also Balanced Annihilation, Tech Annihilation, Zero-K (which is probably the one with the biggest player base atm)

(and others who I think aren't played as frequently, but may be wrong there)