A vector format. This is best if you need to resize the clock (like making it large enough for a banner) without losing any image quality.
It’s a small detail, but if your clipart shows 10:10 (the industry standard for "smiling" clocks), it often feels more aesthetically pleasing than a random time. Conclusion clock clipart
If your presentation is blue and grey, don't use a bright neon green clock unless you want it to be a jarring focal point. A vector format