i recommend the sirio 3 port engine seems to be the fastest engine ive seen for the money
i got mine for 150 and it screams
next engine on my list is the novarossi s3
a friend of mine was lucky enough to get one already and its very fast he told me all things are pretty much maxed out to maximum legal specifications already
mt12 is a great engine and so is mr12 ive had them and they are both great but sirio is faster IMO
i have never had a RB engine but ive heard nothing but good things about them
there are alot of good choices out there nowadays a couple years ago it was mostly mt12 engines