The last couple of times I've purchased batteries for my K's, I'd gone with Westco 12V20P(I'd generally get 5 years out of them)... However....
My new to me R1100, that uses the same battery as the K1100, and came with the same said Westco brand battery, yet still seemed to need more starting umph that what the Westco was wanting to put out and it was less than 1 year old.
I'd get ABS low power faults almost every morning at startup.
At the recommendation of numerous R bike owners, I went the route of a Motobatt AGM battery. Purchased thru Tom Cutter (Rubberchickenracinggarage.com) with 24mo warranty. From Tom it was $140 +S&H.
https://www.motobatt.us/MB51814-Motobatt-12V-BatteryI've yet to leave the garage with it as the snow doesn't seem to want to stop long enough........ but so far I'm impressed with it's performance. No trouble starting and no ABS faults at startup, even when cold.
The last Westco I bought for the K's were back in 4/19 so we'll see how long they last this time around. I will more than likely go the Motobatt route when I'm due for a battery for the K.