If you need a high-performance engine, go for the Nova Flash Tuned, RB T10RC etc
For local racing or just plain having a good time, any engine is good

The RB T10 Junior is a fantastic engine, but don't expect it to be all powerful and mighty.
The STS engines are great for cheap engines, they shouldn't be underestimated.

If you do want a good engine that isn't too expensive, Id look into these ones:
RB T9/T10 Juniors
Novarossi NS12 (or something like that)
Picco Evo 4 (heaps of torque)
The STS lineup you speak of
OS 12TZ/XZ
GRP Tuned/Non-tuned engines (very fuel efficient)