i.e. the largest, smallest, deepest, farthest, earliest, weirdest, exitiest... all the extremes.
n.b. this gets subjective in places because what exactly is a roundabout anyway, and will also have gaps, omissions and inaccuracies, but thankfully a lot of my readers are inquisitive research-friendly pedants.
The largest
London's largest roundabout is the CranhamInterchange, aka M25 Junction 29, an intersection with the A127. It's the M25's easternmost junction, indeed it's right on the edge of the capital and Essex kicks in as soon as you join the A127 slip road. It's also almost perfectly circular with a diameter of 225m (roughly the length of two football pitches), which means if you were to drive all the way round it'd be a 700m circuit. As a motorway junction it's not especially pedestrian friendly which is why I don't have a photo of it, although I have walked round it while trying to cross the A127 in Cranham and it was fairly traumatic. I do however have a photo of the second largest.
London's second largest roundabout is the PoyleInterchange, aka M25 Junction 14, an intersection with the A3113. This time it's on the very western edge of the capital and most of the slip roads lead straight into Slough or Surrey. It's 220m wide but not quite circular so more like 190m wide on the perpendicular axis. As a motorway junction it's not at all pedestrian friendly, but they did have the foresight to route a footpath through the middle should anyone have a desperate urge to walk from Stanwell Moor to Poyle. The path dips down to a remote-feeling subway, then climbs to cross the motorway beside the inner edge of the actual roundabout, then dips down to a remote-feeling subway again. Both halves are planted with thick woodland, in one of which I disturbed teenage moped riders who thought nobody would ever find them in there so I left pretty sharpish.
London's third largest roundabout is the BrentwoodInterchange, aka M25 Junction 28, an intersection with the A12. It's 220m long but only 160m wide and has three slip roads that lead into Essex. The largest roundabout that isn't on the M25, nor on the very edge of the capital, is the BecktonRoundabout where the North Circular meets the A13. It's 210m at its widest dimension. Fifth place goes to the Waltham CrossRoundabout, aka M25 Junction 25, and I can confirm that walking through that is a much more pleasant experience than negotiating Poyle.
The most easterly/northerly/westerly/southerly
Interestingly we've already mentioned three of these. London's most easterly is at M25 J29 (Cranham), the most northerly at M25 J25 (Waltham Cross) and the most westerly at M25 J14 (Poyle). The roundabout at Poyle includes the westernmost point in the whole of Greater London. The most southerly roundabout is harder to determine, the M25 being of no help. I think it must be the mini roundabout at the foot of Old Lodge Lane close to Kenley Airfield, unless there's a roundabout further south in Biggin Hill I haven't spotted.
The smallest
I'm not sure it's possible to determine London's smallest roundabout with any degree of certainty. It's probably a mini-roundabout somewhere, but short of measuring them all (which would be impractical and dangerous) the tiniest will remain unidentified. There might somewhere be a smaller object that traffic gyrates around, like a lamppost or a milestone, but would that even be a proper roundabout?
The most exits
This is a great question - which London roundabout has the most exits? Most have three or four, some have five and a few have six. But I reckon there's at least one roundabout with seven exits and you already know where it is, it's this.
This is Seven Dials in the heart of Covent Garden. This seven-way junction came about in the early 1690s when Thomas Neale realised he could squeeze in more buildings if the streets met in a series of triangular wedges. Originally all seven streets had different names but they've since been rationalised to become Earlham Street, Mercer Street, Monmouth Street and Shorts Gardens. The sundial in the middle has only six faces, which might seem remiss but the argument is that the column itself casts a shadow creating the seventh dial. And if you're thinking "that's a monument in the middle of the street, that's not a roundabout", I draw your attention to the blue roundabout sign on Monmouth Street which confirms yes, it absolutely is.
Roundabouts I've spotted with six exits include the Leamouth Roundabout (near East India DLR), the Gallions Roundabout (near Gallions Reach DLR) and the Rosehill Roundabout in Sutton. There may be more. I should say I'm not including gyratories, nor garden squares, nor residential roads that just happen to be circular. I nearly included Hyde Park Corner on the list but on closer inspection it's more of a complex road junction than a proper roundabout which follows proper roundabout rules.
The fewest exits
One exit would be silly, more a turning circle than a roundabout. It is possible there's a partly-opened roundabout somewhere that thus far only has one exit but I haven't found one of those. Roundabouts with two exits are equally silly, because what's the point, but I have found a particularly good example and it's here.
This is the CyprusRoundabout on Royal Albert Way, an elevated dual carriageway running parallel to the Royal Docks. It runs above the Beckton branch of the DLR, indeed was built at the same time, and in two places incorporates a roundabout with a DLR station in the centre. At Beckton Park that roundabout now includes access to the failed Royal Albert Dock development, but at Cyprus there is no such access so the roundabout still has only two exits. It's proper ridiculous. The first roadsign you pass as you approach in fact shows a double bend, not a roundabout, because this 'junction' has proved to be an unintentional means of stopping cars from driving too fast. The approach also has a sign saying 'New Roundabout Ahead', despite it having been here since last century. Cyprus may not be the only two exit roundabout in London but it is the stupidest.
The oldest
I suspect this comes down to definitions, but Seven Dials being 17th century probably makes it London's oldest roundabout.
The oldest mini roundabout
Now this I can tell you, thanks to a detailed online tribute to the inventor of the mini roundabout, Frank Blackmore, who died in 2008. He was a WW2 RAF pilot who rose to the rank of Wing Commander and then in 1960 started work at the Road Research Laboratory in Berkshire. His first dabble with a smaller than usual roundabout was at Peterborough in 1968, but his brainwave that you could simply paint a circle on the road was first brought to life in South Benfleet in May 1970. Upton Cross (in Dorset) followed in June and then in July he introduced a double mini-roundabout at Eastcote (along the High Road near the tennis club) and that's still there.
The weirdest
Frank Blackmore was also the man behind the Magic Roundabouts in Swindon and Hemel Hempstead, the extraordinary amalgams of five or six small roundabouts to create a larger one. Neither of these are in London but we do have a similar one and it's to be found on the eastern edge of Heathrow Airport, just outside Hatton Cross bus station. The Hatton CrossRoundabout is formed from five mini roundabouts arranged in a pentagonal loop, each joined by a two-way road whose lanes are divided by a line of wands. Two of the exits are merely service roads so it's not especially busy, but it does seem to work well and without the trepidation Swindon might cause. Why drive that far for Magic when we have it here in London?
The most layers
This is my local roundabout, the BowInterchange. Not only is there a roundabout at ground level but also a flyover (for the former A11) and an underpass (for the current A12), making it a triple-decker junction. This is rare. It's not unique in London because the Cranham Roundabout also has a flyover (for the M25) and an underpass (for the A127), but Bow does feel like the most compact example with a top, middle and bottom.
And those are London's most extreme roundabouts. Unless of course you know better...
• The double decker: Westferry Circus, Canary Wharf (thanks Dave)
• The double flyovered: Staples Corner (west) (thanks Sprout Eater)
