I have a great deal of experience with barbiturates and will chime in here.
I've done Vinylbital, Thiopental, Secobarbital, Pentobarbital, Amobarbital, and Phenobarbital. Potency is in that order. No, this isn't a joke.
[A few years back I also did a joking take on a barb I called Flynnal. This is because my name LOL. I wanted to create a bit of an entertainment for everyone. That stuff is not real, despite my claiming it was. It was just for a laugh. Hahahaha but the other barbs including Vinylbital are indeed real]
The thiopental (pentothal) beat pentobarbital, no contest and with a much quicker onset of action. The amobarbital wasn't that much stronger than phenobarbital, but it was definitely a bit more euphoric. This is one of the weaker oral barbs, with secobarbital and pentobarbital being widely known as the strongest oral barbs (that is, until you compare them to Vinylbital)... Secobarbital beat pentobarbital in potency and was close to thiopental and the euphoria lasted a lot longer with no major hangover effect.
The most potent barbiturate of all was Vinylbital. That one was absolutely the king of barbiturates and it's very toxic as well apparently. And before someone says that Vinylbital isn't real I'd invite them to do some Google-Fu.
My experience with Vinylbital was quite recent (2015) and one that totally changed my opinion on Nembutal and Seconal and knocked them off their perches, both of them.
As far as oral barbiturates go, Vinylbital is King, Seconal is Queen, and Nembutal is Prince. Vinylbital is about as potent as thiopental so it's more potent than Seconal. From my experience it could be the most potent oral barbiturate on the planet. That shit really, really kicked my ass for sure. I almost couldn't believe how strong that shit was. Immediate inebriation. IMMEDIATE inebriation. God awful hangover too. Took 200mg and when it first hit me I thought I was going to pass out, it was that potent. Woke up gasping for air twice whilst I was asleep that night. But that was my fault entirely because I forgot I'd taken it and then drank one 375ml bottle of standard strength beer a few hours later which was a really dumb idea in hindsight - you should never drink alcohol if you've taken barbiturates, EVER. A good idea is to get all booze out of your house if you've taken barbs, because of the nature of these drugs you might even forget you took the damned thing.