What is the best place inside Office 365 to store MXF files which have sizes between 35-50 GB

Would suggest considering OneDrive or Azure Blob Storage, in short, simple deployment becomes to OneDrive while long-term storage or advanced media workflows, may go to Azure Blob Storage:

OneDrive for Business

• Supports Large Files: OneDrive supports files up to 250 GB, which comfortably accommodates your MXF files.
• Ease of Use: OneDrive integrates seamlessly with other Office 365 apps, making it simple to share and collaborate.
• Cloud Storage: Similar to SharePoint, it’s part of the Office 365 ecosystem, so permissions and management are unified.

Azure Blob Storage

• Designed for Large Data: Azure Blob Storage is ideal for storing large, unstructured files like video and media files.
• Scalability: It offers extensive scalability to handle vast amounts of data without performance degradation.
• Advanced Media Handling: Azure also integrates with Azure Media Services, which can help manage, encode, and stream large files.
• Cost-Effectiveness: Azure Blob allows you to choose different storage tiers (hot, cool, or archive) based on how often you access the files, optimizing costs.