White Labs and Wyeast both claim to be “pitchable” yeast. I think White Labs started that first. Wyeast still comes with the activator pack that you are supposed to smack in advance. The old rule of thumb was to allow one day for every month old the pack is for it to swell up. Wyeast changed their packaging some time ago and they now print a use by date instead of a packaged on date, so this is a little harder to figure out these days. People here say its not necessary to smack the pack. It just provides proof that the yeast is viable.
I’ve mostly used Wyeast and I used to own a homebrew shop. Occasionally I would find a pack that fell to the back of the fridge under something and got lost. For fun I would smack them and use them if they came up. I had at least one pack that was 2 years out of date and it swelled after 10 days or so. I’ve never had one not do it.
These days there are some new players like Imperial and Omega which are in bigger packs and claim to contain even more cells. I have a best bitter going now with Imperial Pub, which will be my first time using yeast from either of these companies. The pack I got was out of date by several months though so I had to make a starter. It came up quickly, even after the pack had gotten half frozen. I imagine if you had an in date pack from either of these companies you could brew spur of the moment.
Liquid yeast has traditionally required some planning. But I’m like that anyway. I have my next 4 brews already planned and will not have a problem changing my schedule around within reason.
I have avoided dry yeast. When I started brewing almost 25 years ago dry yeast was not good. I tried dry yeast recently, and I still didn’t like it. I tried one that came highly recommended by several brewers off one of these English ale threads. And I wound up with 2 badly infected English ales off it - following my normal procedures I do all the time. These are the same recipes I’m re-brewing now with Pub.