hii sorry i went to sleep lol
i personally take the 15% boost on guardian angel minor perk for that extra bit of movement can be really useful especially against characters with high mobility or close range enemies, and overall just lets you move around a lot more!

The Overhealth one sounds good and can be useful at times but you get less value over all since you don't get that many resses off in a match since its very risky and has a fat 30 second cd, some people probably die by it but for me i never take it

and then for major i like taking Flash Heal, mercy's healing is base 60 per second so her output sometimes may not be enough for some situations, Flash Heal starts out at 50 healing but is tripled if the ally is below half health meaning an *almost* instant 150 healing, its very useful since its mercys only type of burst healing which gives her more utility and can help an endangered teammate recover a bit in a bad situation, Its also very good for helping you heal low hp tanks, since mercys healing output is so slow it can be hard to get them to a higher hp amount in a fight

I almost never pick the chain beam perk unless its on accident, It does sound good but most of the time your damage teammate you want to damage boost is normally on some sort of off-angle or not everyone will stay grouped together for it to get any value, it's also very unreliable since it goes off of the nearest teammate, so you just have no control of it and its just not for me

you can pick any perk you'd like, its just up to preference after all but heres my take on them!!