First ScotRail.
Yeah yeah. Most respondents to this thread will go 'why?' It's for 3 main reasons, arguably 4.
1: I have no real memories of the National Express era of ScR barring a few hazy memories - these include my only Class 303 rides - from my early days (I was 8 years old when they lost the franchise, to give you some idea of my age), so I can't really form a conclusive opinion on them one way or the other.
2: It was during their tenure that I started travelling by rail independently on a almost daily basis, mainly when I started attending college in central Glasgow in 2012, and on the whole they got me to where I needed to be in good time, so a good number of pleasent memories there for me.
3: I feel Abellio was something of a step backwards, at least in terms of reliabilty/punctuality. Granted, the Edinburgh - Glasgow Improvement Programme really kicked into high gear after they got the keys to the franchise, which didn't exactly help, but my overall view of Abellio ScotRail isn't quite as positive looking back. The shambles that was the Class 385 cab screen issue that forced the 314s to stay on much longer than initally planned didn't help either and enough has been written about the Inter7City HST saga to fill several tomes.
4: The Strathclyde/Central Belt network saw major changes and improvements during the FSR era in terms of rolling stock and infrastructure: Airdrie - Bathgate, Larkhall branch, Maryhill - Anniesland & Gartcosh all reopened, Paisley Canal, Whifflet-Coatbridge Central & Springburn - Cumbernauld all electrified, the Class 380s ordered and put into service (despite initial teething problems), Glasgow Central gaining two new platforms (granted they were for the cancelled Glasgow Airport Rail Link, but still), the Shields Junction-Paisley Gilmour St being tripled/quadrupled (also for the cancelled GARL project), to name the biggest/most noteworthy projects. E: Barrhead - Kilmarnock was also partially re-doubled in this era (2008-2010 specifically) to allow for a half-hourly service in each direction with Dunlop & Stewarton both being rebuilt with a second platform added.
ScotRail was one of the better rail franchises that First operated, all things considered.