Investors have flocked to Y Combinator’s Demo Days for years to get their hands on promising startups building cool tech. After all, the accelerator has produced some of the biggest tech companies in the world, from Airbnb and Reddit to Dropbox, Zapier and Stripe.

That’s why we make it a point to keep an eye on the event to spot the most interesting companies from each batch. As I’ve been doing nearly every quarter now that the accelerator has moved to four cohorts a year, I asked nearly a dozen investors which startups were most in demand at Y Combinator’s Winter 2026 Demo Day earlier this week.

To ensure our list included truly sought-after standouts, a company had to be flagged as a ‘fave’ by at least two different venture capital investors to make the cut.

As for valuations, I’m hearing that at least a couple startups have raised funds at a $100 million price tag, though notably, those startups are already bringing in run-rate revenue of $1 million or more. Even for the less buzzy startups not on this list, the “default” valuation this quarter seems to be around $30 million, which investors told me is roughly two-fold the current seed market average.

Without further ado, here’s the list:

Beyond Reach Labs

What it’s building: Deployable solar arrays for satellites.

Why it’s a fave: The startup claims it has developed solar arrays that are the size of a dining table at launch, but unfold to the size of a football field when they reach orbit. The founders say their system can increase available power ten-fold while slashing costs by 88%. Beyond Reach already has a flight planned for 2027, and says it has secured $325 million in letters of intent from leading space companies.

Byteport

What it’s building: A ridiculously fast file transfer protocol.

Why it’s a fave: According to Byteport’s founder Jayram Palamadai, existing file transfer protocols like TCP are too slow for the AI age. That’s why he built DART, short for Dynamic Accelerated Record Transfer, which can apparently transfer large files at an average of 10 times faster than TCP, and even up to 1,500 times faster on “reliable connections.”

Hex Security

What it’s building: Continuous AI-powered security testing tools.

Why it’s a fave: To fight hackers using AI to launch non-stop cyberattacks, Hex is building AI agents that can act as penetration testers, constantly probing for vulnerabilities and security gaps in companies’ infrastructure. By automating what was once a manual process performed infrequently, Hex claims it can prevent attacks at a fraction of the cost. The startup claims it has crossed run-rate revenue of more than $1 million in just eight weeks, which may be why VC investors, as one person told me, “were fighting” to invest in the company.

Grazemate

What it’s building: Autonomous drones to herd and monitor cattle.

 Why it’s a fave: Moving cattle on massive ranches is an expensive and dangerous undertaking, often involving helicopters and motorbikes. GrazeMate’s founder, who grew up on a 6,000-head cattle station in Australia, saw a way to make life easier for ranchers, so he dropped out of college where he was pursuing a robotics degree.

GrazeMate’s drones can automatically guide cattle to different areas of a ranch, estimate animals’ weight, grass availability and growth, and can follow pre-specified route plans.

GRU Space

What it’s building: Permanent lunar infrastructure, starting with a hotel on the Moon.

Why it’s a fave: “Humanity will become interplanetary. It’s a matter of not if, but when, and the time is now,” says GRU Space founder Skyler Chan, a recent Berkeley grad who previously built software at Tesla and worked on NASA-funded space tech.

Chan claims his startup has developed a “moon factory” that can turn lunar soil into structural bricks, which he plans to use to build a luxury hotel on the moon as a “wedge” for broader lunar infrastructure. GRU’s astronomical aspirations, including its goal to open the first lunar hotel by 2032, have made it one of the most talked-about startups of this YC batch. The company has already secured $500 million in letters of intent, an invitation to the White House, and even a reservation from the Trump family.

Luel

What it’s building: A marketplace for human-captured data to train multimodal AI.

Why it’s a fave: Founded by two UC Berkeley dropouts, Luel is building a data marketplace that connects AI model makers with contributors who can submit “daily-life” activities, such as ironing or patient-doctor conversations, to provide audio, video and image data. The company claims it’s generating ARR of nearly $2 million within six weeks, fueled by high demand from robotics and voice AI labs.

Pax Historia

What it’s building: An alternative-history strategy game powered by AI.

Why it’s a fave: Pax Historia allows users to rewrite history in a way traditional strategy games can’t. Using generative AI, the game responds to infinite, complex geopolitical scenarios, from “What if Rome never fell?” to “What if the USA took over Greenland?” The founders claim the game currently attracts 35,000 daily users who have played nearly 20 million rounds.

Stilta

What it’s building: Agentic AI for intellectual property and patent lawyers.

Why it’s a fave: Stilta’s founders claim that patent disputes can cost up to $4 million per case, largely due to manual document review costs. The startup says its AI agent can search and analyze patents across databases and scientific literature, saving both time and legal fees.

The company’s agents are already being used by IP lawyers at pharmaceutical giant Roche. For investors, another attractive aspect is that the founders hail from Sweden — recent Swedish successes like Lovable and Legora have created something of a “halo effect” around companies from the region, one VC investor said.