tl;dr at the bottom ;)
As far as I'm concerned Aguero has only had 2 spells where he's been at his peak level, in terms of BOTH goalscoring and all around play.
The first 4 months of the 2011-12 season, and the first 6 months of the 2013-14 season.
Both things have something very obvious in common. A good strike partner.
In 2011-12, he struck up a very strong working relationship with Dzeko, as well as Tevez briefly (about 3 games before Tevez disappeared and about 3 after he came back. Although that made up about half the games Tevez was around for)
In 2013-14, he struck up a phenomenal partnership with Negredo.
If you look at his Atleti days, he was at his best playing alongside Forlan.
In general, Sergio has always played his best football alongside a skillful target man. I feel this is because it allows Sergio to get on with the stuff he knows best, running at defences and scoring goals; whilst the target men focus on bringing everyone else into the game. Given how well rounded both Forlan and Negredo were, it's no surprise to see Sergio's best form come alongside these two.
But there have been in my opinion three problems, that have really started to take its toll on Sergio.
1) The 2012-13 season. Before this season he was a player you could rely on to last a full season. But from here he started to get his recurring knee and hamstring problems. Now the injury record itself isn't the problem. In fact in recent years he's kept himself in good shape remarkably well. But it's definitely taken its toll on his explosiveness. I always say, look at the goal he scored in the 5-1 away at Tottenham. That was a raw, fresh and explosive player. We don't see that anymore. The link should start from said goal.
2) Playing alone up front. Given his best form has always come next to a target man, it's no surprise he's struggled a bit on his own. The fact is, he's not a player who can play this role. It's extraordinarily uncommon for a small out and out striker to excel playing on his own up front. The only exceptions tend to be players with exceptional strength, link up play, or excellent goalscoring wingers up front with them. In the later Pellegrini era, and obviously under Pep, Sergio has had to play up top on his own. Playing the role of primary link up, hold up, and goalscoring player. It's a testament to his ability that he's done as well as he has given he's not a natural in said role. It doesn't help that he lacks the goalscoring teammates around him (eg David Villa, Pedro, Eto'o, Muller etc). A new problem in this area, is that Gabriel Jesus ticks a lot of boxes here. He is an excellent link up player, very good at holding the ball and has a goalscoring knack about him. He's not the complete article and still a weaker player than Sergio, but he is a better fit, per se.
3) Confidence. The first two problems alone add up to plenty here but I hardly think he's helped by the media. I've said it plenty before about several of our players (particularly Sterling), but when the media gets an opportunity to destroy a City player they don't tend to hold back. There's been a developing narrative since 2013 that he's a thug of sorts, and up until the point he actually got dropped last January the media were asking questions of him. Admittedly, as much as I love Pep, he didn't help with his comments earlier in the season that Sergio could do 'more'. Ever since Jesus has established himself, the media haven't stopped peddling the Sergio out stories, peaking with the ridiculous player + cash swap suggestion with Sanchez. When Sterling fluffs his lines I put it down to a lack of confidence ahead of stupidity. It's the same for Sergio I believe.
You just feel he's a hattrick away from exploding again, and honestly I look forward to it. I'm confident it'll happen this season.
One thing's for sure. Sergio is a City legend. He's always given his all in every game. He's always fought for his place, even when he was a guaranteed starter. Last season when Pep demanded more from him, he gave more. Last season when he was dropped, he came back fighting more than ever.
Whilst it hasn't reached the levels of Zabaleta, it's clear that he's slowly but surely declining. But like Zabaleta, we can all be assured that he's going to commit every last drop of blood, sweat and tears for the cause.
Sergio is one of my all time favourite players. Not just for what he's done, but for how he's entertained and dazzled us throughout the years. I'll never forget that bonkers goal he scored against Liverpool a few years back (February/March 2013?). But I think with the emergence of Jesus we should be excited for the future, and accept that maybe Sergio's days (with us) are somewhat numbered. Let's enjoy what's left. And pray to Jesus (the religion one, not the striker or the winger we had) that nothing bad happens so that he can nab those few extra goals in order to take him to all time top scorer. The QPR goal alone merits such a feat.
tl;dr Things have been tough for him, he's done really well regardless. He might be on the wane, especially with Jesus emerging, but we should enjoy what's left and encourage the best out of him.