Team Fortress 2 is (was?) a significantly better game. And I love Overwatch. It was my #2 game of the year and I still pop in every now and then while I haven't played TF2 in years.
A very easy one is simply the maps. You could write a book on how brilliantly designed many of the TF2 maps are. Gravel Pit, Badwater Basin, Gorge, Gold Rush, HIGHTOWER etc. The best maps are designed where all classes can excel. There are no pointless areas and since TF2 is so situated around traversal, having multiple options when it comes to attacking and defending is vital and keeps matches dynamic. Overwatch has some good maps, but none I would call brilliant. Some are just straight up frustrating, like Hanamura.
From an aesthetic viewpoint, TF2 is largely superior (inb4demopan). It took a very bold premise, mid-twentieth century commercial art mixed with 60s spy/science fiction, and managed to develop an amazing cohesion among the maps and character designs. The art design was brilliant. Even though many of the maps took place in similar and outright boring settings (farms and factories), they were visually distinct due to their impressionistic design. TF2 went down the rabbit hole a long time ago adding in crazy costumes, but initial look for the game was so daring. Overwatch has a pretty safe art style and is a crazy mishmash of every sci fi trope you can think of. I think the game looks great and there are some amazing character designs, but the direction they based TF2 on is so bold to me.
Another thing that immediately stuck me as being inferior to TF2 in Overwatch is how the characters interact with each other. TF2 is filled with so many amazing quotes and voice lines that happen naturally in the game, but Overwatch is seriously lacking in that department. There are some clever equippable voice lines, but I almost never hear people use them. The same goes with sprays. Yeah, people spam sprays in base, but in TF2 people used so many creative sprays and loved to show them off. Perhaps this is a side effect of allowing custom sprays.
I also think the general flow of a TF2 match is better than in Overwatch. So many Overwatch games end in the last minute when one team finally gets their stuff together and decided to pop their ults at the same time. If TF2 you had to make more deliberate decisions. You needed to snipe a specific person at a specific time, synchronize members to take down a dreaded sentry, take unorthodox paths to flank the enemy etc.
Team Fortress 2 is one of my favorite games of all time. I'm talking about definitely in my top ten, if not top five. It is the best online experience I've ever had. Playing on the TF2 GAF server are some of my fondest gaming memories, and I still talk to many of the people I played with to this day. Overwatch is Team Fortress 3 in a lot of ways and I'm thrilled that someone made an incredible TF2-ish game. It's a logical evolution of many of the things that make TF2 great while also incorporating fresh and exciting ideas. But to me, Overwatch still doesn't touch what made TF2 phenomenal. Maybe one day it will though...