I use orange seal but have no particularly loyalty to it or ever compared it to anything else.
But for putting in sealant through the valve stem, definitely get the KOM cycling injector. It’s not that expensive and it makes it super easy and less likely to clog the stem since you inject the sealant right into the tire. It also makes it easy to remove the sealant from the tire before you go to remove it.
I have had terrible luck with Stans on my road bike. I am generally running at pressures around 70-75psi and every puncture i have had witt Stans has not held until the tires were half flat and it alway seems to blow out once the tires are re-inflated.
I have had good luck with Orange Seal, but regular and endurance and most times i dont even realize i had a puncture until i see the sealant on the frame after the ride
The ironic thing about all of this…
I decided going tubless was an inconveniece, so purchased some TPU tubes to give a go (ive been using latex tubes for the last 5+ years, with great success) the tubes arrived in the post.
Now over the years I (thought) id become an expert at fitting/removing tyres. I often fit difficult tyres for a couple of buddies, as they struggle or cant fit tight tyres themselves.
Well, there is no way I can remove these GP5000 S TR’s from my wheels, its just not happening. Not without damaging my new wheels anyway.
So ive actually been forced to go tubless 
untill these tyres need replacing. And when that times comes, I think ill have to cut the tyres off.
Anyone have an opinion between Stans Race and Orange Seal Regular in terms of what seals better on MTB?
(Edit: looking for the best race day sealant, not super concerned about longevity)
I do like to be able to inject through the valve because I’ve had some issues with out-of-round tires, so I like to be able to mount them and seat them, and make sure everything is good-to-go before filling with sealant.
It seems like Race would seal better but it is designed for race day scenarios where not getting a flat is a high priority. The downside is that it’s gooey, not recommended for installing through valve stems, and dries out quickly.
With exceptionally stubborn tires, I have resorted to using angled pliers with a cloth between the jaws and the tire so as to not damage the tire.
Or just take them to your LBS and have them do it. 
Should have been more clear, was thinking for a race day type sealant. Longevity isn’t my biggest concern in that case.
Silca, then Stan’s Race
Stan’s for every day, Stan’s Race for race days. So far I’ve had two issues, both on same course (Badlands Gravel Battle).
First was due to a rock slicing and there being no sealant left (my fault as the sealant was absorbed by brand new tires and I didn’t check before race day). Held for 2+ hours and then required 2 CO2 canisters and 4-5 top-ups to finish the remainder 3hrs of the race.
Second is a head scratcher - flatted, aired up to 25-30psi so I could make it to next aid station, aired up to 40psi and it held for next 5 hours. However, I subsequently ound 5-6+ tiny holes through the tire after that didn’t seem to seal no matter how much sealant I dumped in.
This is across three years, two people riding, multiple road/cx/tri/gravel races, 7-8 bikes, 1500+ hours on various road, CX, gravel and MTB tires from 25mm upwards (includes my partners bikes which I maintain). Compared to easily a dozen flats per summer between two of us when riding tubes …
using Stan’s Race in both road and gravel with very good results. FWIW, you can actually add Stan’s Race through the valve using a sealant syringe, it’s just a royal PITA. I use a KOM syringe, thinking of trying the Park tools one instead
I’ve definitely seen bits (whatever they’re made of) in the Stan’s race bigger than the ID of a valve, so if you are installing it that way you’re not getting the benefit of the bigger bits. FWIW, I often set up wheels with the race, then top up/keep it liquid with the normal Stans. Theoretically, the bits won’t go anywhere…although I have had them all join together and make a large ball rattling round the rim before!
I’ve had a lot of success with Stans race. Never had a puncture that didn’t seal.
What I did find was that it didn’t work well with cotton cased Vitoria tyres though. Even small holes didn’t seal well and the sealant clumped on the inside of the tyre.
I’ve just dug the Vitorias back out to try with Silca ultimate, to see if it’s any better. The only trouble with Silca is that it stains clothes whereas stans just seems to pick off like rubber.
Stans question, race or not. Does it always smell like ammonia? I bought a used bike in October and it came with Stans and S-Works Pathfinders. Cleaned out the Stans and put Orange Seal Regular. Love the tires.
The one thing I’ll say about Orange Seal… you really have to shake it to get the little plastic confetti stirred around. Thought I had a problem with Specialized Mondo tires, turned out all the confetti was at the bottom of an old bottle of Orange Seal.
Tried Silca Ultimate with the bike’s stock wheels and after a month the valve stem was clogged. So I took a metal wire that is just slightly smaller than the valve, and bored it out. However I was still unable to fill the tires as it seems Silca somehow did a great job sealing the inside of the tire near the valve stem. Life is too short, so I hung up the heavy Bontrager stock wheels and put my Roval Terra wheels on with the Pathfinders.
So far I have used Silca, stans, orange seal, and muc off. I had issues with stans in cotton tires as mentioned above for sealing leaks. Silca is great when it works, my issue is that 1) it clogs valves terribly, 2) it seemed to dry up in the tire faster than other brands. Muc-off was a disaster. Couldnt even seal small holes and I got covered in pink fluorescent stickiness since the tiny hole it finally sealed kept re-openning while riding. Orange seal has been great so not sure why I ever switched.
From Stan’s FAQ: “ Any trace amount of ammonia in Stan’s Tire Sealant will evaporate soon after it is injected into the tire. We have found no damaging effects from our sealant even after years of use.”
But does it always smell like ammonia when removing a tire? Super sticky too, made the mistake of using old leather yard work gloves and it took 2 months for the sticky to go away.
Quick Q. Few days ago I went to the bike shop to get the sealant refill. Yesterday, before the ride, I wanted to check the tire pressure and when I’ve tried to inflate, the sealant started to leak, and I couldn’t inflate. Am I doing something wrong? 
Possibly, unless they filled the tyre completely!
You want the valve at either 4 or 8 on the clock face (so, sideways but slightly downwards) and leave it a few seconds for any sealant in the valve to drain before trying to pump. Then unscrew the valve and tap the end to release any sticky sealant, and you should get either nothing or a tiny mist of sealant coming out.
Thanks for the suggestion, Alex! 
Would be better to put in the 12 position or 3 or 9?
At 12 the valve would be almost guaranteed to be full of sealant (unless you’d left it at 4/8 beforehand to drain, in which case that would work) I do the above if ever having to use CO2 roadside, to give it the most distance to travel before hitting/potentially ruining the sealant.
3 or 6 would work, but might not drain as fast.