There are a number of good levels out there. I recommend a scope-mounted level because some of the rail-mounted levels are, well, not level!
I like the Accuracy 1st DG level, but it's made currently only for righthanded shooters and I'm a leftie. I had to return the two levels I bought, but I'm hoping they will come up with one for us lefthanded shooters. I like it because the white ceramic ball seemed easier to see than a bubble. It's not top or back lit by sunlight, which makes bubbles easier to see.
I have a couple Flatline OPs levels on order. I bought them based on SH recommendations, but I got the "fixed" and not the "fold-out" model" that is twice the price, which varies according to the scope tube diameter. I found their pricing a little nutty.
The Holland looks good, and the Vortex is low cost and well regarded too. I also have the new Tubb scope level, but I haven't shot with it yet. It's small, lightweight, and unobtrusive, can be mounted high or low, and has a window to let sunlight in to better see the bubble. David has a video about this new level on his website.
If price is a major consideration, then go with the Vortex.