Probably virtually every overdrive out there. I've tried almost all of them and haven't liked any of them. I've tried the HBE Hematoma, Tubescreamer TS9DX (analogman modified), GK Diesel Dawg, EHX English Muff'n, Sansamp BDDI, Ashdown Drive Plus, Humphrey modded Bad Monkey, DOD Preamp/Overdrive 250 and the list goes on and on. The Fulltone is about on par with most of these IMO but I liked that it could be powered with conventional DC power and didn't take up a ton of real estate. My second favorite was the DOD 250 but it was inconsistent (sometimes you would get breakup and sometimes you wouldn't) and it had a crappy bypass and staticy pots.
I think the problem might lie with my bass though because the Stingray puts out such a hot signal, I think I prematurely clip the FETS. I am in love with the Ray's clean tone so I just have to compromise and accept that I might not be able to get a satisfying Overdriven tone. I currently use a Fulltone BassDrive which is as subtle as I could find. It works but still isn't what I had in my head.
My current favorite pedal is a EH Nano Small Stone. Such a nice texturing effect that I had never considered using on bass previously.