[Someone1234]

For all of its flaws I will say Google Docs Spreadsheet.

While it doesn't replace Excel fully, you can use it and almost forget you're using a "webapp." The cells/columns look great, feel great, and they almost nail the desktop experience down to a tee.

It is actually a fairly old webapp, but I still haven't seen anything which made you forget that it was a webapp quite that well. Even GMail/Outlook.com still feel like webapps even with how good they both are design wise.