Silver Dollar I would give a miss, I think Vegas Technolgy is the weakest of the software and support is not great.
Rival some people like but I find it far too streaky to be trustworthy, that said it is apparently possible to win in the short term though I never managed 1 single cashout at any Rival after about my Second deposit.
As I played less my bonus offers got bigger and my returns got smaller,fancy.
RTG some excellent slots IMO with good support (right Casinos) and some decent bonus offers but returns have been questioned more and more lately and I no longer play any RTG even though these are my favourite slots.
KK mentioned Wizzard gaming and they have some fun slots these Days just don't expect the software to play consistently.
They have potential though.
Grand Virtual have some fun slots but support is generally poor to very poor.
Micro Gaming Systems have a massive selection of games and usually excellent support. some people have not enjoyed much success on the slots after recent updates to the software.
Playtech have some reasonable games and good selection but stay away from the new novel slots. They may play decent for a while but sooner or later you will hit a streak of death that never ends.
Be careful where you play too, just like RTG there are lots of iffy Casinos running the software.
In short if you play slots, play for fun and take a bonus as near to +ev as you can get.
Cashout when you are infront and play elsewhere for a while after you do.
These Days that is your only hope of gettiing value for money.
Have fun