It's really really hard to pick, man! their voices are just so unearthly, and they've sang in hundreds and hundreds of movies! If they're only looking to hear samples, then yea, youtube is best. A couple of good songs:
Lata: Ajeeb Dastaan Hai Ye: http://www.youtube.com/watch?v=eJLdL5_QkVo
Asha: Aaiye Meherbaan: http://www.youtube.com/watch?v=urUpMevl_XQ
(ignore the subtitles, they get more and more inaccurate as the song goes on
Movie recommendations for Lata's best old-era singing:
- All the songs from Mughal-e-Azam (Pyaar kiya to darna kya, Bekas pe raham keejiye (http://www.youtube.com/watch?v=HGzCS4C8-UE
), a movie in which Lata's voice is said to drip honey)
- Pakeezah (ALL the songs in this one. Inhi Logon Ne for instance or mausam hai aashiqana http://www.youtube.com/watch?v=ARHnwslf9Uc
- Chalti Ka Naam Gaadi (what to say about this movie
- movies by Laxmikant-Pyarelal as the composers usually.
- Vande Mataram in Anand Mutth demonstrates her control over the notes: http://www.youtube.com/watch?v=xj1Iy4nRMkc
- Personally, I thought Lata did a great job with Luka Chhupi from RDB (http://www.youtube.com/watch?v=iB-PMhP7NRk
) but I wouldn't call that her best singing of course
- Teesri Manzil,
- Yaadon Ki Barat (the famous Chura Liya Hai Tumne: http://www.youtube.com/watch?v=ApOIJZrFenU
- the extremely well-known Hare Rama Hare Krishna (songs like hare rama hare krishna or dum maro dum)
- the old Umrao Jaan. Superhit!
- movies with R.D.Burman as the music composer. Not only is the music smashing but the songs are great too.
A movie that has both of them singing is Shree 420. For instance, Mud mud ke na dekh was sung by Asha (http://www.youtube.com/watch?v=5EBrkEyCJ5Q
), and Lata sang the preceding opposition to that song: O Jaanewale Mud ke dekthe jana (http://www.youtube.com/watch?v=vy6jtwxvS_c
I think both of theirs old Marathi songs are the best where their voices were young. Their voices were sublime! Here's an example: Asha http://www.youtube.com/watch?v=oD8AxzgVbeA
(Brahma Vishnu ani Maheshwar from Aamhi Jaato Amuchya Gaava) or Lata: http://www.youtube.com/watch?v=5jrBq2qt-DU
(Akhertsa ha tula dandavat - you can lose yourself in this song, it's like reaching nirvana or finding god or whatever
) or singing the Ganesh aarti (http://www.youtube.com/watch?v=PN1C4XODKWY
. in this one the video is new but the track is from the much earlier).
Bottomline: hard to pick. But hope you have some from the ones above.