For me the Gears franchise was amazing with the first game and became a little less interesting with each game. I went from loving it, to liking it, to not caring, to hating it and now I'm just back to not caring. This series just isn't for me anymore.
Resistance was good but never reached it's potential. I think a reboot would do very well on PS5. Maybe after Spider-Man that would be something they'd want to do.
Killzone games are well-made but felt forced because PS3 needed an exclusive FPS. Like Gears, I doubt I'll ever play another Killzone tbh.
I'd rank them:
1. Resistance - Of the 3, this is the only one I have any interest in playing going forward.
2. Gears - Good games, but I'm over the series. Hope the studio moves on to new IP eventually.
3. Killzone - We have Horizon now. At this point, Guerrilla is too good for Killzone.