I always prefer fountain pens, but I do need to jot quick notes on the fly for work, so I got a bunch of higher end rollerballs/ballpoints. Almost all of the ball pens out there are sized for either the Parker G2 ballpoint refill or the Schmidt rollerball refill. There are a few brand proprietary ones like Cross and Montblanc, and some annoying model specific ones like my Aurora Thesi.
For ballpoint, Montblanc is the smoothest I've found and is also pretty reliable and has decent capacity. The Fisher space pen refill is the most reliable I've tried and will work after years of not being used, but it's not too smooth and the line is ugly and blotchy.
For rollerballs, my favorite is the Pilot G2; smooth, reliable, and inexpensive. The high-end German-made metal refills seem to go dry pretty quickly in storage. The Pilot G2 is supposed to be a Schmidt size refill, but it doesn't fit in some high end rollerballs because the thin section at the grip end is thicker than on a metal refill. I can get them to fit in my Visconti rollerballs by whittling down this section a little, though.
Then there's something that's kind of a hybrid and my favorite find so far: the Kaco G2 refill. This refill is Parker G2 sized, so it will fit in almost all "tactical" ballpoint pens as well as high end twist ballpoints. But it's actually a rollerball refill in ballpoint size. It feels similar to the Pilot G2, a little finer and not quite as reliable. But they work most of the time and when they do, they're very smooth. They are also very inexpensive.