I've got most of their manual focus bodies now (thanks to jumping on ebay auctions which were finishing cheaply or freshly-listed low buy it now prices).
I would say the K2, KX, MX, and P30/P30n/P30t.
I started with a K1000 but the KX and K2 are much nicer bodies to use. They have a more accurate meter, better viewfinder display (although I wish the K2 had the aperture display of the KX - I know the K2DMD does but prices for those in working order are mad), self timer, mirror lock-up, DOF preview, etc. If you don't yet know what to do with any of that you can still shoot them like a K1000, the controls and meter display are just as intuitive (when the black needle is on top of the translucent needle then exposure is correct). The only problem with the K2 is the film speed setting which is renowned for being awkward to use. Mine's not too bad and I've seen suggestions involving a dab of silicone grease on the moving parts to make them easier to use.
The MX adds user changeable focussing screens and the option of adding two types of winder and a databack, plus it's smaller. It doesn't have mirror lock-up but the LED light meter display is easier to read in poor light, although in good light I prefer the KX/K2 display.
The P30 series are black sheep. They're derided as plastic and usually cost less to buy than a crate of beer. Yet the chassis are diecast metal, the viewfinder displays the shutter speed, they have wonderfully simple controls, they're light and very unobtrusive. A P30n or P30t with a 35mm prime makes a brilliant street camera as it doesn't shout "mug me" and you have Av mode for grab shots (the AE lock button is more useful than exposure compensation too).
I'd suggest a P30n or P30t over a K1000 for someone wanting to try film as they can get a fully working example for very little money, the metering is more accurate, and they can be used as a fully manual camera. The lack of an override for the DX coding is probably a bit annoying if you're into pushing or pulling film but you can either remember to under/overexpose to taste or use tape to alter the contact strips on the film cannister.