I put single Mangekyo Kakashi on the same general level as alive Minato. He may lose in a one vs. one confrontation against him, but if pit against multiple opponents, I see them performing roughly the same in the long run. Kakashi has a versatile, more lethal arsenal and a more flexible S/T jutsu in Kamui (self teleportation and long range teleportation). Minato has the arguably faster teleportation ability in FTG, but is lacking in offense.
Double Mangekyo Kakashi is unquestionably above alive Minato, and should be comparable to BM Minato. He still lacks feats, so it is yet to be seen, but Perfect Susano'o has paralleled Kyuubi avatar, and double Kamui is far more useful than Hiraishin. Again, we lack feats so all this is yet to be seen.
Overall, MS Kakashi ~ Minato's tier (again, speaking general level rather than one-on-one confrontation), and double MS Kakashi ~ BM Minato's tier.
Minato? The bastard summoned a dead god, sealed the fox inside him and then through his sealing leet skills made the fox's chakra seep into him.
This is a suicidal technique. BM was never achievable in life because the moment he activated the reaper, he signed away his life. The only reason we got a BM Minato is because by some miracle there was someone out there with his DNA that was also able to revive him. We also don't know how Minato magically became best friends with the Kyuubi and skipped the whole "internal battle" with it like Naruto / Bee did (unless Naruto beating it made the half in Minato friendly as well, which makes sense). Alive Minato deserves BM just as much as alive Itachi deserves Kotoamatsukami; it was a power available to them under very unusual conditions.
It's why I tend to separate alive Minato and BM Minato due to this.
It's -ALL HIM-. He never had a KKG or some awesome gifts. Everything he did was through his own efforts.
It isn't really a fair comparison. Kakashi had the Sharingan thrust upon him when he lost his eye in battle. He then had to attribute and train his body to accept a foreign Kekkei Genkai, and we see him gradually improve with it throughout the entire series. It isn't fair to call the Sharingan a free power-up; Kakashi worked hard to make it usable over the span of 18 years. It initially drained him and he worked his way up to the level he was before Madara snatched it.
Kakashi had the Sharingan for pretty much his entire relevant life as a ninja. It makes no sense to randomly snatch it away from him when he has had it for 18 years and then measure his power without it and say "he sucks without Sharingan." He centered his entire fighting style around it. He was already a genius at age 12 / 13, was a Jonin, and was improving rapidly. Had he never received the Sharingan, he would have centered his fighting style around something else.
But yes, I agree with you if we're talking the double Mangekyo and Perfect Susano'o. That was 100% completely gifted and out of the blue. But Kakashi deserves the left eye because he worked with it throughout his entire ninja life, and it wasn't easy; it was draining at first and only became more usable as the series went on.
Were Kakashi to invent some Lightning FTG or an awesome jutsu of his own that can trump Hiraishin, maybe. BUt as long as his strongest jutsus are copied/received... meh.
Minato didn't invent Hiraishin, Tobirama did. Minato "copied" Tobirama's jutsu.
As I said earlier, I agree with you if we're talking about double Mangekyo Sharingan because Kakashi never had to work for the right eye and the process of unlocking Perfect Susano'o - it came out of the blue. But Kakashi is more entitled to the left eye than Obito was; he had it for 18 years and worked through the trials and tribulations of becoming accustomed to a foreign, draining Kekkei Genkai.