Depends on your budget. $49.99 with a trade in gets you a MECOA .46 (a clone of the old OS 46 SF) I have 3 of these and if broken in properly (like any engine) you can't beat the price for the amount of engine. Next up the money scale is the GMS 47, a strong running engine. My next choices are the Irvine 53 and ASP/Magnum 52. My ASP 52 will turn an APC 12.25 x 3.75W at 12,200. My Irvine is a little slower with this prop. I have just broken in a JENN 56 and still like the ASP 52 better. The JENN sure looks like the Tiger Shark which was the low end line conceived by Jim Goad of Indy R/C fame (I believe he later sold to Horizon Hobbies) BTW, the high end engine in Jim's line was the GMS series. My "ultimate" "Rip The Firewall Off" engine, however, is the Jett Bar Stock 76L. Although pricey, this is the best quality engine I've ever owned. A favorite business saying is "I would rather explain a higher price once than make excuses over and over for poor quality." The bolt pattern is the standard OS 46 size. This compact, and light power house is designed for larger props, not high RPMs. Mine turns an APC 14 x 4W at a comfortable 10,000 RPM. On a Sig SE this wide blade prop will hover at 1/3 throttle.
I love tinkering with the 2-strokes and have found almost no engine over the years that I couldn't get to operate correctly.
My list is not exhaustive by any means. I have not yet tried the MVVS 49, but understand it's a real powerhouse.
Scan this site for all the posts on engine break-in and also go to Jett Engineering for great information. With a truely effective break-in just about any engine will be a winner.

There are some extremely knowledgable people on engines on this site. Personally I read every post I can find by our friend in Israel, Dar Zeelon. I think he has more knowledge about engines in his little finger that I will ever have in my whole brain.