I started on stars, went to FT for bonus + RB, and have now moved to UB for bonus + RB.

Stars: Love their software the best, most players, tons of tables. With proper table selection you can find decently fishy games. Shame about the no RB or bonus though.

FT: Software is OK, a little worse than stars. Less players, but still easy to fill 9 tables at micro-low stakes. Maybe a touch fishier than stars but not by much IMO. The bonus clears at a reasonable rate. I completed a $500 bonus after 30k hands at 50NL & got the friend bonus.

UB: software is ass. So many irritating things about it. Doesn't save window size, throws a dozen annoying popups at you when joining tables and you have to keep the HH window open for your HUD/PT to work. Bonus is slow as molasses to clear. Maybe 1/3rd the rate of FT's bonus. However, they have a huge bonus ($1k if you can grind it out). Less players, but holy piss are the tables fishier. Very Easy to find 30% flop / 20BB avg pot size at 50NL. Such tables are far harder to find at FT/Stars in my experience.