Czech loads (often identified as "M48") are supposed to be the hottest. However, Winchester (which in this case is just repackaged Sellier & Belloit) and Prvi Partizan both equal this, firing an 85 grain bullet at around 1600-1650 fps. I was able to find the Prvi for about $15 per 50 at my local gun show.
I've shot two cans (roughly 2,400 rounds) of the Romanian so far with pretty good results.
I've had bad luck with the Bulgarian and Czech. Primer blowouts were the main problem. The Czech I had was 50's vintage, can't recall what age the Bulgo was exactly, but it was early cold-war also.
The Bulgarian may have been worse than usual as the can I got had the seals compromised, probably ages ago. The dinky little paper wrappers had degraded into purple and tan wood-pulp, and the ammo was discolored. Smelled like chemical eggfarts when fired. Horrendous. It will choke you out if you get a lungfull. The other commie 7.62x25 smells terrible also, but that Bulgarian will clear an indoor range in a hurry.
Within the same brands, or across brands (countries)?
It's pretty well-known that the rounds vary by country/year/brand, as the round has a commercial loading, military loading, hot loading, and a machine-gun loading, although I doubt you had any of that. If it was within the same brand, box, or can, then you have a point.
The Romanian stuff from Aim has been great for me. I'm on my 4th big can now. The primers may be a little hard sometimes so I put a tiny 5 cent washer under the hammer spring as a spacer and have had nmisfires in the last 3000+ rounds.