It's not flashy Leica glass, but you can't beat the original SMC Pentax (no -A or -M) series for a combination of price and quality. Pentax produced several lenses that, while slow, surpass faster lenses even wide open You can either "spend" lens groups for speed, or you can spend them on correcting various abberations. I can confirm that my SMC Pentax 35mm f/3.5 is a gem (as is my medium format SMC Pentax 6x7 75mm f/4.5, and my SMC Takumar 35mm f/3.5 that preceded it, I'm virtually certain both are direct copies). I just picked up a SMC Pentax 28mm f/3.5 on the same impression. I haven't shot it yet as it needs some cleaning, the aperture is a bit slow.
Pentax's Super Multi Coating is a major boon. It virtually eliminates flare and was DECADES ahead of its time. After the patent expired virtually everyone (that is, everyone who hadn't licensed it already) quietly copied it. They also were way ahead of the game with regard to normal lenses - the Takumar 50mm f/1.4 was definitely in the upper tier in its time and still holds its own today. I don't think there was even another 50/1.4 SLR lens at the time. Maybe Nikon had a pre-AI lens out, but it'd be close.
The Pentax ME is an incredibly underrated camera. It's a 1.0x (actually, 0.97x or something) magnification SLR with a standard split-prism focusing screen. Compared to any other SLR I've handled, including my K1000SE, it feels like the same difference as between an IMAX screen and a movie screen. It's bigger, it's brighter, it's better, period. You can buy the original ME for $20-30, it's absolutely the best deal in film photography. Throw a $50 50/1.7 on the front and you'll have an incredible setup. Forcing yourself to use a normal lens is incredibly for your composition skills, just take that one lens out on a walk and you'll feel better about yourself.
The one thing to note is that in my opinion, M42 glass is superior to K mount glass because the K-mount aperture lever is forced wide-open by the plastic square frame inside my Canon 40D. If I rotate the lens to the 'lock' position, the lens is stuck wide open no matter the aperture setting. I think this may be a peculiarity of the adapter, but you have been warned. Also, you should spend at least the $20 a manual focus screen off the auction site will cost you. It will make manual focus ***incredibly*** easier. You could even pay for a Katzeye or a real glass screen, it will probably be even better.
Zeiss, Rollei, or Leica glass obviously needs no introduction. I also absolutely adore my Nikkor 105mm f/2.5, it's extremely sharp wide open.
This is what's left after I crop away 75% of the image, wide open. I simply don't give it a second thought, it's an absolutely amazing lens despite minor pitting on the rear element.