I use a 3/4" straight router bit for this. As suggested in the comments, a surfacing bit is recommended to achieve a much smoother finish.

I position the bit so it will remove about 1/4" of material from the highest point on the log end.

The router sled is used to guide the bit over the log end in careful passes to remove material. I have found that working the sled back-to-front works best, pulling the sled toward me for each pass beginning on the right side of the log and moving to the left. I don't remove any material on the return-to-back pass. This way, the majority of chips and sawdust shoot to the rear, rather than into my face.

However, I still wear a full-faced protective shield for this--the kind you'd use when working on a lathe. This is the one I have, and it works really well.

As viewed from the top of the router, the bit rotates in a clockwise rotation. The type of passes I've described above are called "climb" cuts, which will tend to pull into the yet-to-be removed material if you're moving too fast and not holding the sled firmly, or if you try to remove more than about 1/2 the width of the bit in material with each pass. These are things to keep in mind that will help you achieve a cleaner finish.

Remove wood in layers (I suggest only up to 1/4" at at time, at most) until the top end of the log is perfectly flat.