Which is the "BEST" deep learning model for "Custom" object detection for images & real time. YOLO v3, v4, v5, EfficientDet?
There are many subtle differences between yolov4 and v5 other than speed like YOLOv4 exceeds YOLOv5's performance on the COCO benchmark.this link will help you explain.
YOLOv4 VS YOLOv5.
I dont know much about EfficientDet.
Here's YOLOv3 Versus EfficientDet for State-of-the-Art Object Detection
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It explains quite nicely difference between the two.