From what I have heard, Sloto' Cash is allegedly the safest.
I used to love Rival casino software and played a ton.
Now, I STRONGLY recommend you be careful reading terms and conditions.
Many promotions in Rival casinos includes timed play restrictions, game restrictions that 'void and null winnings', much larger playthrough restrictions, non cashable bonuses (not as unheard of, but a recent change to Rival), and I've heard that bonuses may become STICKY (the absolute worst to watch out for)
It will be a while before Rival casinos have my trust again.
When they first came out, there were many cases where free spins didn't register if autoplay was being used in games like As The Reels Turn... I don't think that it happens anymore but the software can still be viewed as flaky at time.
Oh and Tradition casino ... *shudder* listen to everyone else and avoid PLEASE. Blackjacks paying at 1-1 should be enough evidence there.
Cheers!
- Brandon