Well... first... welcome to AA!
And secondly... we get a lot of these "what are the best" questions all the time. The top of the page has a search button... where you can easily search for posts about this very topic... very likely you'll find at least 30 about this question alone.
Smaller systems really depend on how small. 10 gal
small, or 30 gal
small? Anything would be good, really. CPR bak pak, Aqua C HOB
, even a Sea Clone would work, even though most disapprove of it. I have a CPR on sale right now on the classified section. Depends on if you will have a sump or not... but many "smaller" systems are sump-less.
But first... take a minute and search the past threads to get ideas on where to start out. HTH
. Let us know if you need help with a specific question.