metagross

Oh yeah, Metagross with meteor mash is a beast! Do the lucky first (costs less stardust and it has really great iv too!) and then see how many dust&candies you have left :D

Speaking as someone who's got 15 metagross (I had 1.3k candy going into beldum day, buddied a beldum for 6 months waiting for this, the others I get from mass distance trading with friends before CD weekend) More than one powered up one is fantastic. I got three big weather boosted one with good iv's, and they are monsters.
Best part is the auto select favors tanky pokemon, which means when gym battling, your metagross are almost always top pick, and is great for sweeping out those low motivation or even normally motivated gyms, and metagross also functions great when placed in gyms.

Depending on your play style, it really can be that useful. I've got 2 of them maxed out (one lucky and one shiny), the rest sitting at L20 or L30 as they were hatched or caught during CD.

For raids, not a great investment at all. However, for gyms as they have mentioned already, auto-pick places it in front of (you guessed it right) Aggron, so no fear to hit the green button. I still prefer Machamp leading the party most of the time in my area, but the game insists on putting my "Gyms" party as one of the last ones and I don't always have the patience to look for it.

For PvP, I'm not sure yet, it is still pending to prove its value, but I don't PvP that much lately.

Do the lucky to 40. The more expensive one you could stop at 35 in the near future, then max it in the long future. However, I’m not a huge Metagross fan, my 15/14/15 lucky is still at 35. I have a few at level 30, plus one lucky shiny at 1499 and two at shiny at 2499.