In the spreadsheet realm, we're seeing an intense race to determine which features people want most and to get those features pushed out the door. Overall, I tend to favor Google Sheets, not because it has a broader set of features than Excel Online (it doesn't), but because one feature -- programmability -- can trump them all. Your results may well vary. If you rely on Excel formulas and features such as pivot tables, you'll undoubtedly side with Excel.
For presentations, Keynote for iCloud easily outpaces the competition, with Google Slides pulling up a solid second. PowerPoint is undoubtedly the weakest of all the online apps. It seems that the designers of PowerPoint Online looked at Keynote for iCloud and said, "OK, we give up."
All nine apps are changing, very quickly. In recent weeks, Apple announced that Pages could now export in ePub format, Numbers could export to CSV, and Keynote has added a setting to show or hide slide numbers. All iWork for iCloud apps now support up to 100 people collaborating in a document at the same time. There are 200 new fonts, more color options, and new interactive charts.
Also in recent weeks, Google announced it had new antitheft account verification, and it had completed the rollout of direct image editing (anticipated in my Google Drive review). Microsoft doesn't have the hell-bent-for-leather update pace of the other two, but in April the official Office Blog talked about a bunch of improvements in Excel Online.
If a particular app doesn't have a feature you need, check back again in a week or two.