In that case, I'd determine the size you need, find some reviews on longevity, and balance all of that within whatever your budget is, and get a drive. Personally, I'd go with 1TB or 2TB drives, as those seem to be the sweet spot for what you pay for, currently.
When you're connecting over USB2.0 with a modern hard drive, then the connection will be the limiting factor, NOT the hard drive.
A USB 2.0 connection will generally give you 30 to 35MB/sec top speeds.
Current mechanical hard drives easily have throughput rates 80 - 100 MB/sec minimum. Well, they may be less than that in real transfers on occasion, depending upon the various components in between, size of files, how much stuff is on the hard drive, the speed of the other drive, and anything else related..
Also, the RPMs are mattering less and less in mechanical drives. For instance, the new Samsung F4 2TB drive appears to be faster, overall, than the previous F3 1TB drive. The F3 is a 7200RPM drive, whereas the F4 is a 5400 RPM drive. The access time (from tests I've seen online) is maybe a hair faster on the F3, but the total read/write speeds are far greater on the F4 than the slight diff of access times on the F3.
Anyway, all that to say base your decision moreso on warranty, dependability, user reviews, etc, than raw speed - b/c you won't see the speed benefit over USB 2.0. Oh, and if you want any certain features, look for those.
Personally, if it were me, I'd look for something like this:
USB and eSATA connections, at least 1TB, 3 year warranty if possible, etc.
Now I can suggest one drive that is actually sort of a small RAID enclosure of 2 Samsung hard drives.. It's by Lacie, I think...
I don't remember the model number or name right now, but I picked it up for a client a while back. It's used as a back-up drive for them, and I put it in a RAID 1 configuration. That way if one drive fails, or the interface fails AND their computer fails, they still have their data... so long as at least one of those drives doesn't fail.
One big thing about that drive was I got a good deal on it at the time AND it came with a 7 year warranty. Seems a bit excessive for a hard drive compared to all others I've seen to date.