Is Slowbro a good defender?

In the current meta anything can be brought down with modest effort, don't invest big into defense.

I have battled them, and they are tough. And that moveset is a good one. Yes, it's a good defender. As an attacker, I'd rather battle a P/HB Blissey than a C/IB Slowbro.

Vaporeon is poor on defense for its weak defensive Quick move, and Lapras for its relatively low attack stat and weakness to the ever-present Machamp (which in the past were in short supply and had a poor movepool). Slowbro has a great defensive moveset of Confusion/Ice Beam, but its attack is only slightly better than Lapras and its defensive stats aren't fantastic.

It's good for keeping a gym solid and for awhile, as it disrupts Machamp/Dragonite/Fire-type sweeping and its relatively low CP will last awhile, but I definitely would not MAX one. You should really only be maxing attackers currently.

With the current system of gym decay, placing in high CP Pokemon like something maxed is only super useful if 1) You're trying to hold a gym during an EX-Raid or 2) if gym turnover is extremely high and you really need coins.

If you're looking for something to MAX and you can already power things up to level 40, then you should really wait for a better mon. A bad Blissey or Snorlax will certainly fill this role better than a Slowbro will. Slowbro isn't the best mon for super active gyms, and low-turnover gyms don't really necessitate MAXed mons, even if your Slowbro is 100% and the best example of a Slowbro.

If your aim is to halt Machamp/Dragonite/Tyranitar sweeps, then you should consider looking into Gardevoir for gym defense. Its tankiness is slightly under Slowbro's, but it boasts much higher attack, far better typing, and a more dangerous Charged Move. They're normally about as rare as Larvitar but can be found in numbers during both Cloudy and Windy weather.

If you're really insisting on the Slowbro (Which I wouldn't blame you for either), I would power it to level 30 and stop there.

You can stop level 30-35 and spend half as much stardust.

Slowbro is one of the good choices for placing in between two bulky normal/ice/rock/steel pokemon that are weak to fighting.

The better question is the status of the gym. I have a gym I take over almost every day, and am able to get my 50 coin almost all the time.

Find a similar gym near you. Gym defense is only good for 8 hours and 20 minutes. Of course, you could raid 50 gyms a day, and hope they stick for at least 10 minutes, but that’s a lot more work for what it’s worth.

I alternate between my Snorlax named Gym Rat and/or a fleet of Blisseys. They get stuck sometimes, but they earn or combine for 50 almost every time I use them. Looking forward to adding Slaking to the rotation.

Check out Gamepress’ Meta Analysis of Gym Defense. It’s pretty sound information.

No defender is worth leveling to level 40. The only time one has any business pumping a defender to 40 is for trolling/bragging (ie, take down a gym and stick out your tongue at the other team by putting a level 40 exotic thing in there. Someone did that with a hitmonchan way back in the day fairly often). 30~35 level range is fine.

I would consider taking the 100% slobro with ideal defending moveset up to level 30~35 and putting him in gyms like that (they are annoying to fight) - but no further than 35. IMO it's better to have a selection of decent defenders leveled 30~35 not one at level 40, so instead of just putting individually great mons in to defend, you can pick mons that are annoying for an attacker, based on what's currently in there - ie, follow a blissey/ttar/lax with something that punishes fighting types, something fairy type if there's a miscelaney of unimpressive defenders, to annoy the person trying to sweep with a 'nite, etc. So the attacker has to do more work :-P

Chansey is particularly nice since you can get one up to around 1,000 cp (minimum decay rate) without much investment (IMO, a low-attack-IV one pumped up higher to get ~1k CP is better than a higher-attack-IV at lower level - the point of chansey is to decay slowly, and die slowly in battle)

Slowbro is a horrible matchup for Machamp and Ice Beam is a threat to Dragonite, so it's a pretty good defender. Gets eaten alive by Tyranitar, but forcing the attacker to change or faint their Machamp is really what the whole thing is about.
I would not put dust into getting it to 40 though, Blissey and Chansey, maybe Snorlax are the only mons worth dust for their defensive value. Catch and evolve a level 35 Slowpoke when it's windy or rainy and you have a fine thing to stick between normal types.

Yes it is good but dont go over level 30.
Thats enough for a defender atm.

Slowbro and Slowking are one of the top choices to defend a gym after Bliss, Lax, King and Chansey.