We've used Sixt several times and had no problems. The one time we went with another company...it was a major name but I'm not positive which and I don't want to demean anyone incorrectly!...we got socked with a damage charge for something that had been in place before we left and was misrepresented (the window in the back of a van that opens about 2 inches to let in air had a bad latch...they listed it upon return as a "broken window"--yikes!). Anyhow, after that, I'd definitely stick with Sixt.
I can also second that Odysseys is iffy...we had a standard class manual rented from them a few years back and they didn't show up at the airport for the scheduled pick up...we finally reached them and they said the car had been in a wreck and they only had that one, sorry, which was weird because we weren't picky on class, etc. and it was shoulder season, not high. The main thing is that they didn't let us know...they just didn't show up. From reading reviews about them afterwards (I know, probably should have done that beforehand), though, it looks like we dodged a bullet.
Incidentally, I've rented from local companies on Crete for years and they have been great...it's just this particular one I've had issues with. :-)