(げんかく) A hallucination – whether a pink elephant or an abstract geometric form.   $$$
(まぼろし) Like 幻覚 but more broad – it can be sounds, tastes, etc.
(げんそう) A vision – a hallucination that goes on and on like a movie. Like the English word ‘vision,’ 幻想 is often used to tell the future or help making a decision.
(げんし) An illusion you see when everything else is totally normal. Like if you see a 幻覚、 you’re tripping balls and EVERYTHING looks weird. But if you see 幻視、 that is more like you’re walking down the crowded street and you catch a glimpse of your Grand-father ... but he’s been dead for 10 years. But you could swear it was him!