The Stadler/3rd-gen stock on the Glasgow Subway replacing the Met-Camm/2nd-gen stock comes to mind with light rail/metro. It certainly seems to have gone more smoothly than both the Tyne & Wear and Merseyrail upgrades/replacement programmes, regardless of - or perhaps because of - the Subway's self-contained nature.
On the heavy rail side of things in Strathclyde, the 380s taking over from 318s & 334s on the Ayrshire Coast & Inverclyde springs to mind. Granted, the latter is a lottery these days with every current ScotRail EMU type barring the 334s appearing on at least one out/back diagram every weekday, but the former has been solid Desiro territory since 2011 and has by and large been all the better for it. Part of me still thinks more 380s should've been ordered for the E+G, but that ship sailed years ago.
Once they had eventually settled down and the laundry list of problems gradually ironed out, the 334s ousting the 303s directly or the 314s through cascades.
Going back even further in time but still sticking with Strathclyde, the 303s/311s replacing a mixture of steam-hauled stock and DMUs, which were themselves a big step-up, even if their Gresley pattern bogies made them rather lively ride-wise, especially when at speed.
Scotrail 385s on the Edinburgh-Glasgow corridor providing more coaches and an EMU service, which presumably also helped cascade 170s to strengthen DMU routes.
Certainly in terms of overall capacity increase and passenger amenities, the 385s have been the biggest upgrade that route has seen in at
least 40 years, though in terms of actual comfort, opinions seem to be far more mixed. Personally, comparing the few times I did the route when it was solid 170s compared to the hundreds of miles I've racked up with 385s this year alone on the route, I can say that me and my backside would prefer a 170, regardless of the major capacity boost or the fact the journey time on this route often as not comes in at just under a hour.
Most members of a certain age/generation would more than likely cite the 47/7+Mk3+DBSO sets as the high watermark for passenger comfort on this route, though given it was long before my time, I can't really pass comment on it.