How does the F-35 stack up against the competition today? That depends on who you ask and how badly they want to buy/sell it.
First off, let's leave the stealth business out of this. Like most emerging technologies, it's brittle and subject to being overcome or otherwise rendered obsolete.
One important aspect of any fighter aircraft is maneuverability which, in dogfighting terms, means the ability to escape being shot down by your adversary but shooting down your adversary instead. In general terms this is a function of power, thrust to weight, and wingloading. On this score, the F-35 is mediocre at best.
Thrust to weight - Rafale, 1.13; Eurofighter Typhoon, 1:12, F-22, 1.2; Mig29, 1:12; F-35, 0.83. Anything 1.0 or greater means the aircraft's thrust exceeds its weight, meaning it can go straight up if necessary. It's also an indication that the aircraft can probably achieve "supercruise" - the ability to cruise at supersonic speeds without having to resort to fuel-guzzling afterburners. The Typhoon, Rafale and F-22 are all known supercruisers. The F-35 is not in that league. As the numbers show, the F-35 is really sub-par on thrust to weight.
Wingloading measured in kg/sq. metre. Rafale, 304; Typhoon, 300, F-22, 342; Mig-29, 411; F-35A, 446. Once again the F-35 finishes a solid last. In other words, with the F-35's poor thrust to weight performance coupled with its high (poor) wingloading, it simply won't turn with the opposition meaning, in a dogfight, it's dead meat.
The RAND Corporation concluded long ago that, placed head to head with any top competitors, the F-35 couldn't "out turn, out climb or out run" them. That, however, doesn't mean the F-35 would ever have to do any of those things. In fact it's supposed to be able to use its advanced electronics to avoid that sort of dogfight challenge. Whether that's realistic is another matter.
When the United States introduced the legendary F-4 Phantom the Americans also believed the day of the dogfighter was over. The F-4 would rely on its (for its day) advanced electronics and long-range Sparrow missiles to do its killing. And then along came Vietnam and America learned it couldn't always dictate how it would fight and that the F-4 would have to dogfight and that having been designed without a cannon left it at a mortal disadvantage.
The F-35 is intended to play to its strengths, stealth and advanced electronics. But the other guys play that same game. They try to match up their strengths against the F-35's considerable weaknesses. They look for ways to make the F-35 fight on their terms. Every F-35 advantage they can neutralize enhances the significance of their own performance advantages.
Potential adversaries can and will make steady advances in their own radars and avionics and missile technologies but the F-35's shortcomings on power, agility and range are engineered into the aircraft and will probably remain until the 35 is retired 36-or more years after it enters service. That's an awful long time when you're starting already in the hole.