When I went hunting for a couple lenses to fill my prime gaps I grabbed the Samyang 35 and 85 f/1.4's. They're both manual lenses (with 'A' settings), but I mostly shoot old manual stuff anyway so the learning curve is somewhat less for me with them.
I'll go anther route and suggest a good wide range zoom (18-135 pops to mind). Use it for a while, then go back and look at what focal lengths you gravitate to with it, then go for your primes based on that.
Thats why I grabbed the two I did, I tended to shoot 35 and 85(ish) a lot with my zooms, and I lacked primes in those lengths.
As far as lens speed goes, faster tends to be better. This isn't because you want to shoot the things wide open, its because most lenses when stopped down an exposure or two tend to get sharp. This means the 1.4's will be sharp at f/2.0. where that 2.8 will get sharp at f/3.5. That a BIG difference.
Of course the ability to shoot night shots with shorter exposures helps as well. I experienced that just the other night when the aurora decided to show when it was 20 degrees out with gale winds. My Sigma 10-20 f/3.5 failed me because the wind was strong enough to buffet the tripod, and 3.5 was forcing exposure times in the ~15-30 second range.
I popped my 35mm f/1.4 on, same settings save for aperture, and was able to get my sole keeper of the night because the wind ebbed for a moment and I snuck a shot in wide open (f1.4) in under 10 seconds.
Again, lens choice really depends on what you intend to use it for.
Speed isn't everything, unless it is. Weight isn't everything, unless it is. Cost isn't everything, unless it is. Figure out what your needs are, set yourself criteria based on that, then do your hunting around.
Also, don't be afraid to buy old and cheap if you are fence sitting on a lens. See if the focal length works for you, then upgrade. Nothings more annoying then dropping a couple hundred bucks on a lens then winding up keeping it as a paperweight because you discovered you dislike that focal length.
---------- Post added 03-19-15 at 10:28 AM ----------
Originally posted by Dlanor Sekao You could always get a SMC Pentax "F" 35-70 Zoom for about $50 ! I doubt anyone here will argue that its a good lens. Its my latest acquisition and I really really like it ! Plus you get a good pseudo Macro to play with at the 70mm end.....Just a thought
Echoing this. That 35-70 is amazing for what looks like a crap short zoom. Its the only zoom I own I'd place the 'stack of primes' label on.
Thats it in the center. For size comparison, thats the kit 18-55 lens far right, and the abovementioned 35mm Samyang (Bower) looming menacingly overhead.