In stock form and in terms of reliability, economy, and power, my vote goes to the following.
4-stroke is RS EH035 (super quiet, reliable, and fuel efficient. i own 3 of them)
2-stroke is the older Mitsubishi TLE-43, not the newer TU-43.
One of mine was mfg. in 2000 and has more miles on it than i can remember.
it outlasted the original Staton Friction drive kit that it was originally mounted on. i'm thinking over 4,000 miles.
I'm still on the original clutch shoes and just recently changed out the original sparkplug, not because i needed to but because i just thought it was way past due. i bought a spare engine just in case this one should ever fail but it still runs as strong as the first day i had it. it's recently been my daily ride of 25 miles a day, 6 days a week. On my GEBE setup it uses just a tad more fuel than the EH035 with a lot more power because of the Strato-Charged technology. Side by side comparison to my Tanaka PF-4000, it wins hands down in stock form in regards to performance, sound level and fuel economy.
Again, this is only the opinion of someone that still owns a Zenoah GZ25N, Tanaka PF-3300, RS EH035, Tanaka TC-35, Tanaka PF-4000, Mitsubishi TLE-43, John Deere branded Mitsubishi TLE-43, Tanaka TC-47 Type R
(i also own a couple of chinese clones but am too embarrassed to include)