Since you didn't mention what you want them for, I'll first suggest a tyre with heaps of grip.
If you liked the rolling resistance of the fatbike (given you rode Lou, Al Mighty or similar) Schwalbe's Magic Mary is the way to go

. True to size 2.8", painfully draggy, but one of the grippiest tyres on Earth.
I use them on my ssp steel (former HT, now susser,) which I only ride in winter (kind of a training bike) or for summit riding, when almost all uphill means carrying the bike anyways. So mainly situations where I wan't as much grip as possible and don't care about drag.
Incredibly grippy; braking, traction and most of all cornering. If nothing else matters, go with Mary. For the amount of chunk fairly light. I ride it with a 80 g insert (a little puncture protection for the soft alloy rim but mainly to reduce air volume/get more progression from the tyre), since I struggle to find the sweet spot of rebound and puncture protection with this tyre's volume.
Will be trying a 2.6" version once I've gone through the tread or damaged it beyond repair. 2.8" and full suss + insert + flexy frame mutes just a bit too much, but it was what I rode on the HT. Will see how much grip will be traded, but given the abundance, should be good. Go with the lightest possible options (true weight can be found eg on r2-bike.de and bikeimport.ch). Also Soft compound only.
Faster running, but still okay grip wise are Nobby Nics. Looks for 2.6", Super Trail (sidewall version) and Soft (compound).
Fasterer; Wicked Will. Would make a great combo for single track focused MTB; Nobby Nic up front, Wicked Will rear. Speed Grip compound (don't think any other available for 27.5 x 2.6) is kind of okay in the wet, but depending on what and how you're riding, may be disappointing.
Fastererer; G-One. For "grandad MTB'ing"
I just stuck with tyres I know and with one brand, since other options have been mentioned, but for another brand, I can't really recommend Onza unless you ride very wide rims. The 2.6" Porcupine really needs a i40 mm rim to work. Horrid shape on i30 rims and despite otherwise interesting stats, it's cornering grip was very poor unless leaned into the corner heavily.