for a lightweight library then most would go for NotePerformer as it is fairly inexpensive and is undemanding of resources. The BBC SO Core version requires little disk space, is inexpensive on sale but uses more CPU than something of its specs should. I use both (although NP less in Dorico than I used to in Sibelius). When competently programmed – and it’s not the easiest – it can be a very involving and spontaneous performer
Piano. I got PIanoteq 6 a few years ago and was pretty happy. Being modelled it’s easy to play and fairly easy to programme. More recently I have increasingly found the actual character rather tame – it’s well behaved but doesn’t somehow sound like a piano in the lower register in particular . A low budget, modest in resource requirements and simple to use piano can be found from Cinematic Strings. It’s not as deep sampled as their orchestral libraries but you can get quite a variety of tone by adjusting the 4 microphone positions supplied. Although it may not have the character of the greatest sample pianos (which is of course not what you’re asking about), it blends well into chamber or orchestral arrangements. Listen to the demos on their website!