Favorite middle evolution

Honestly, Brionne. I think it is really well designed. Grovyle, Ivysaur, Quilava are neat as well.

I really like Metang. Don't get me wrong I love Metagross, but Metang just does something for me that Metagross doesn't. I like how it floats and just has two little arms (I know Metagross can float, but its typically depicted on all 4 legs). I also like its horn and piercing red eyes. I also have a soft spot for Kadabra, even though I like Alakazam more. As a kid I didn't have anyone to trade with so my Kadbra didn't evolve until much later.

Shellgon. Very silly looking guy, I love him for it.

For me its probably Porygon2. Porygon2 is a fascinating case of unintended consequences! Since it didn't get an evolution until Generation 4, it already had sorta high stats for a middle evolution, and even had a niche in a few formats over Porygon-Z thanks to Trace letting it pull off cool stunts like absorbing Fire-type attacks from Heatran!

But Generation 5 is when things really got crazy thanks to Eviolite! Thanks to that item, Porygon2 actually has bulk fairly comparable to Lugia of all things, and it's been a superstar in many metagames since. Especially the 3v3 singles metagames like Battle Stadium Singles. Seriously, its had a major impact in the singles metagames for gen 6, 7, and 8, and I'm sure Porygon2 will make extremely effective use of Terastallization if it ever comes to Scarlet and Violet!

What impresses me too is how there's been several times where Porygon2 had unexpected twists in metagame developments. For example, in Battle Spot Singles for Sun and Moon, Return Porygon2 existed thanks to Download, and in Sword and Shield, there were actually Iron Tail Porygon2s running around thanks to Max Steelspike make it extremely difficult to KO! And to expand on the previous point, how Substitute + Toxic Aegislash became really effective in Sun and Moon, and a big selling point was "it beats Porygon2".

There's been other middle evolutions that have had impacts, such as Chansey being better than Blissey in some formats, and heck, I even got to Master Ball with Crocalor of all things last season in Battle Stadium Singles (I'm not actually sure it's really worth using, but I have to admit not being weak to Dark was really helpful against Chien-Pao!). But Porygon2 is still in my top spot for middle evolution just because at least in the official single formats, its been a consistent, reliable Pokemon every time I've used it!

I'm gonna have to go with Braixen (pause) Grovyle and Dewott.

These are all rare contenders for midstages that can be considered better than the final evo, not necessarily bc the final evo is bad but bc the midstage is so good.