Not enough info given.

Track size, grip, main lengths 30min? 15 min? 1 hr?, powerband wanted? There is no perfect racing engine. Only perfect engines for certain track conditions.

Best overall, Smooth power, Good fuel economy:
Novarossi +4, OS $peed, RB Buggy 9

Bigger track, less bottom end, more top:
RB WS9, Novarossi Toro Nero

Well rounded middle range:
RB hobby 9

Budget, smaller club racing:
Novarossi N21b, RB Junior 9

Mother of all that is evil Powerhouses:
Novarossi FMS, Clockworks Modded Picco