About the most you can safely put things is to put one pair of H. fuscus in a 20g, preferably with a sump. H. fuscus is a smaller than standard seahorse so with larger more frequent water changes it can be done. Problem now is that I don't know of any source of H. fuscus available in the US, especially true captive bred ones.
As for the quite large kelloggi, they have a VERY poor track record of survival for what has been coming to the US now for some time.
IMO, there IS no BEST type of seahorse, but rather what ones do you like the looks of best, and what types are available from the best source available, seahorsesource.com.
I'm really curious as to why you asked "Can I keep a pair in a 10G or 20G?" after I already posted the recommended tank size?
Did you read the links at the bottom of the "My Thoughts...." page I posted?
There is SO much to learn about keeping seahorses before you get them that one post is not going to be the full knowledge you need. It will take quite a bit of research.
If not wanting to research fully, then seahorse keeping is probably not for you because it definitely will require more work and due diligence than keeping a reef tank.