What is the best mechanical chronograph under 5k?

My opinion… probably Tudor BB Chrono as your also getting something on a bracelet if you stretch definition of under 5k. At release, either of the Top Time Deus or Zoro too, and I really dig the new Top Time Triumph too.

I’d throw in the NG Chronomaster on your consideration list too.

Damasko and Junghans make some pretty sweet looking chronos.

Combat, by mechanical, is it safe to say you mean hand wound? Or are you looking for automatic?  It is an important distinction. If truly mechanical you are looking for some variation of the Sellita SW500 movement, if automatic, largely some version of the ETA 7750. Both having plus and minuses, but if size is a concern the SW500 based watches will be thinner due to lack of rotor. They will also be/feel quieter as every 7750 I have you can feel and hear the rotor.  

- Hamilton Intramatic Chrono is a classic and comes in both automatic and mechnanical.  
- As Bevel said, on the higher end side the Tudor BB is a solid pick up.
- Farer makes some very compelling pieces at the price point.  
- Sinn 103
- If willing to go used, you are now in Navitimer territory, some Speedmasters, Monacos, IWC Le Petite Prince.

in my opinion speedmaster reduced, tudor bb chronograph and oris diver sixty five chronograph

Good deal if you can find a Tudor chrono at that range. Might need to stretch budget a bit. If you can why not look into Zenith? The A384 is also a beauty 🤩 

Longines Spirit.

Kurono Chronograph II White, “Shiro”.

Just kidding, that was gone in less than three minutes. Tudor BB Chrono would be the one I’d be looking at

I'm assuming you mean new - lots of great vintage choices under $5k. I'm also going to assume you mean market price (not list). 

Here are a few out-of the-box ideas:

Zenith Heritage Pilot Type 20 (approx. $4-5k on grey market)

Alpina 130 Heritage Pilot (approx. $2k on grey market - if you want reissue vintage style)

Eberhard Contograf - 1960s Tribute Edition (approx. $4-5k on grey market)

Anonimo Militaire Vintage - Newman dial (approx. $3-4k)

Eterna 1940 Chronograph Telemeter 42mm Dual (approx. $4k)

Favre-Leuba Raider Sea Sky (approx. $4-5k on grey market)

Hanhart 417 ES (approx. $2k)

or you could go for the Revolution Watch limited edition variant (approx. $2k)

Heuer Monaco Gulf Edition (approx. $4-5k)

Heuer Autavia Jack Heuer 85th Birthday Edition (approx. $5k on grey market)

Massen LAB Uni-Racer (approx. $3.5k)

They're all mechanical. Some are automatic, some are manual.

Maurice de Mauriac Le Mans Orange (approx. $3.2k)

Nivada Grenchen Chronomaster Aviator Sea Diver (approx. $2k)

Sinn 158 Bi-compax Pilot (approx. $2-3k)

Yes, they all use mechanics we know that, but in terms of vernacular it is an important distinction; mechanical is often used when referring to hand wound vs automatic (rotor based). For example, the Hamilton Khaki Field Mechanical vs Hamilton Khaki Field Automatic. IWC and plenty of others also use mechanical vs auto.

They're product names. I'd rather be technically correct. 

You be you, but in addition to the companies that actually make the watches using that distinction, you’ll find most of the popular watch sites/blogs/YT channels do the same. Just trying to be helpful to the OP using common enthusiast language. Happy to PM you numerous examples vs use the OPs space.

In this case, to help the OP, the experience between a mechanical (or hand wound) and automatic could matter depending on preference, especially sizing and comfort.

Fun topic. I don't know about "best" but my favorite non-quartz chronographs under $5k include:

I’m agreeing with this - and it makes a difference when owning and caring for it. It‘s a perfectly reasonable question in this scenario.

hanhart 😍😍

Probably not objectively the best, but this is my favorite.

Fantastic choice! Heuer Autavia rules. 💪

My vote is for the Big Eye Avigation Chrono...I am loving it.