If you mean the town then I think it's where the Locke Park tower is. If you mean the entire area then I think it's Howden Edge on the moors above Langsett.
Search the archive, this subject was thrashed out a couple of years ago. In the Barnsley district it will be the foothills of the penines up around Hoylandswaine.
Without heading out to Penistone, I'd say the top of Worsbrough Common - fantastic view across the valley from there as well.
But the true boundary of Barnsley is miles away from there, out on the moors half way to Manchester. There's a little clue out there, a sign saying "Welcome to Barnsley". Howden edge is about 510m above sea level.
Isn't that in Sheffield though? I don't know the Barnsley borough confuses me, driving from Manchester the 'Welcome to Barnsley' sign appears a lot sooner than you'd expect, must be half an hours drive from the town centre.
Usually stop off at Royd Moor viewing point when out on the bike, pretty high up there. If, like Tomfun suggested, you stood on one one of the Windmills, you would be about 357m above sea level! That must be hard to beat, surely?
According to the map Howden edge is on the border between Barnsley and Sheffield. There are many higher points on the moors but none in Barnsley. Highest point in South Yorkshire is Margery Hill 546m but that's in Sheffield.
Hoyland Law Stand looks down on Worsborough Common, it's a good 35 metres higher. That's from the ground not top of the tower.
In terms of Urban areas - the top of Worsbrough Common is 170m. The wind farm is 325m - seems to be highest populated area I can find in the Barnsley anyway. Anyone know where the lowest place is? I'm thinking possibly the bottom of Worsbrough Bridge where the cricket club is - that's about 60m above sea level and flooded badly down there in 2007.
If we're talking about populated areas I'd probably plump for Carlecotes but it's close between that and the windmills. For the lowest point I would go for Bolton-on-Dearne which is below 20m in places.
Google maps has Wuz common at 141m and Hoyland Law at 179, which is said to be the highest point due east to the sea. There are areas of Hoylandswaine 250m+ but it's hard to say where the borough borders lie.
You can see the borders here https://www.barnsley.gov.uk/service...ons/electoral-register/what-ward-do-i-live-in There are several places in Barnsley above 500m, with or without stilts, ladders, high heels etc.
I realise this is not that important, particularly as Sir Bob is about to sign, but the borough boundary extends all the way out on to the moors above 500m. Check the Penistone West ward on the link I posted. Hoylandswaine is in Penistone East.