Can someone explain to me why Mega Slowbro is any good? Everywhere I read, its regarded as a top tier Mega but I don't see the appeal because it loses Regenerator for a near deadweight ability. Getting a crit is so random and low chance that I don't see the appeal except for niche reasons like screwing over a Critdra. I don't really see the large DEF boost as that worth it to forgo something as flexible as Regenerator but most of the community seems to disagree. I would like some insight into why people choose Mega over the normal Slowbro.
Firstly, this is a question better asked in a battle-related section of Smogon. However, I sometimes ask questions of this nature too, so I'll give you a brief and concise answer, expensing on calcs and what not.
In a nutshell, M-Slowbro is a super bulky mon. 95/180/80 is huge, because basically this thing can take physical hits for days. the fact that he's mega means he resists Knock-off which for example, is a big deal. However, MSlowbro's primary functins are either stall, or tank.
So for Stall, Slowbro gets the reliable recovery in the form of Slack Off. Therefore he can keep himself alive whilst PP stalling any big hitting move like Stone Edge taking really minimal damage. MSlowbro gets access to Toxic or Thunder wave. The former move allows him to whittle down opponents and out-last them, the latter allows him to cripple threats that are fast and can't really touch him. HUGE. He also gets scald, so your boy Mega Metagross, unless he carries Grass Knot (which still won't do that much) will likely get burned by scald, and eventually taken out by Flamethrower or something.
For Tank, MegaBro can once again take hits and then boost himself up with calm mind, meaning he already has a base 180 Def, and his special defence will also be boosted immensely. So will his special attack. Slowbro has access to a wide variety of special attacking moves out of which Psyshock, Fire Blast, Focus Blast, Scald, Shadow ball, Signal Beam, Ice beam... they will all do a lot of damage at +2, and this gives Slowbro huge coverage, so a Modest MegaBro with Slack Off, Calm Mind & 2 attacking moves can do a lot. The point of Mega slowbro is that whilst boosting up he will take way less damage from opponents than normal slowbro. Mega Slowbro also gets base SpA boost. Tanky slowbro is good at taking hits, Slacking off + boosting up and hitting back harder.
Basically, Becoming Mega removes any weaknesses non-mega slowbro had bulk-wise (and the reduced damage from knock off... which I want to stress, is huge). Of course he can eventually be taken out, but it becomes an arduous process. And having regenerator means you can use slowbro and switch him in and out to recover health, but once he mage evolves slack off takes care of the rest. It's a similar situation as that of mega sableye losing out on Prankster, except MSableye has less type weaknesses