It may be sacrilege on this website but I do like a twin.
Airheads, Guzzis, Ducatis, Laverda 750s, Commandos etc. Not much bigger/heavier/complex than a single if designed right but a fair bit more oomph. The alternatives of Ducati longitudinal L twins and Guzzis sideways version are especially attractive.
I do like the SV650 and 1000 twins amongst the modern jap engines, though the S incarnation was particularly impressive.
Harleys ? Still trying to make my mind up there.
Richard