I've regularly SLFed on many of the European majors over the last 30 years or so, and I'd say today the answer is without doubt (IMHO) Easyjet. As long as you follow the rules, and maybe splash out the little extra for SpeedyPlus, in my experience they have been unfailingly good. Aircraft clean and comfortable, Cabin staff always friendly and attentive, and a decent selection of hot/cold snacks. My airline of choice for intra-European travel now (not that I do much anymore, since moving to the US)