Now that the dust has cleared from the collapsed rubble that was the 1988-89 television season (which, in case you hadn't noticed, ended last week), it's time to look back on some of the highlights - and lowlights:

Best New Series: "Roseanne" is the biggest ratings sensation, and it is a good new show. But as far as I'm concerned, "The Jim Henson Hour" is the most welcome addition to the television schedule. Let's just hope it can survive a tough ratings start.Worst New Series: So many to choose from here. But I think I'll go with NBC's "Nightingales," with mind-numbing mix of thin scripts, bad acting, medical nonsense and Aaron Spelling-style tawdriness.

Most Improved Continuing Series: "The Cosby Show." What? How can the No. 1 show be considered "most improved"? Easy - the show improved qualitatively, with less pretense and more fun.

Worst Series Slide: This year "Moonlighting" has gotten so bad that I've almost forgotten how good it once was.

Best Television on Television: It was a bleak television year, with the writers strike and the Olympics putting everything on hold for a while. And then along came CBS's magnificent "Lonesome Dove" to remind us how good this medium can be. Yes, it was awfully violent and the language was rough. But you had to admire the artistry involved in every phase of the production.

Worst Television on Television: Another tough call. Was it Geraldo Rivera being hit in the face with a chair? Was it Phil Donahue in a dress? Was it Linda Kozlowski asking her "date" to tie her up in "Favorite Son"? Was it ABC asking us to really believe a romantic relationship between Robert Mitchum and Victoria Tennant in "War and Remembrance" in November - and then asking us to believe it again next month? Was it "The Sex Tapes"? Was it "Naked Lie"? Was it "Live-In"?

I don't know. I don't even want to think about it. Feel free to fill in the blank yourself.

-FOR THE RECORD, the A.C. Nielsen Co. says the top 10 programs for the year were:

1. The Cosby Show (NBC); 2. Roseanne (ABC); 3. A Different World (NBC); 4. Cheers (NBC); 5. 60 Minutes (CBS); 6. Golden Girls (NBC); 7. Who's The Boss? (ABC); 8. Murder, She Wrote (CBS); 9. Empty Nest (NBC); and 10. Anything But Love (ABC).

The second 10 consisted of: 11. Dear John (NBC); 12. Growing Pains (ABC); 13. ALF (NBC) and Matlock (NBC); 15. L.A. Law (NBC); 16. Monday Night Football (ABC); 17. Hunter (NBC) and Unsolved Mysteries (NBC); 19. In the Heat of the Night (NBC); and 20. Head of the Class (ABC).

The rest of the A.C. Nielsen ratings list looked like this:

21. Night Court (NBC); 22. The Wonder Years (ABC); 23. One of the Boys (NBC), The Hogan Family (NBC) and Sunday Night Movie (NBC); 26. Amen (NBC) and Monday Night Movies (NBC); 28. Knots Landing (CBS) and Sunday Night Movies (CBS); 30. Dallas (CBS) and Monday Mystery Movie (ABC).

32. Full House (ABC); 33. Jake and the Fatman (CBS) and Designing Women (CBS); 35. Nearly Departed (NBC); 36. Murphy Brown (CBS); 37. Monday Night Movies (ABC); 38. 227 (NBC) and Sunday Night Movies (ABC); 40. Family Ties (NBC).

41. Perfect Strangers (ABC) and Midnight Caller (NBC); 43. 20/20 (ABC) and My Two Dads (NBC); 45. thirtysomething (ABC); 46. MacGyver (ABC); 47. Mr. Belvedere (ABC); 48. Kate & Allie (CBS); 49. Moonlighting (ABC); 50. Newhart (CBS).

51. Nightingales (NBC), Father Dowling Mysteries (NBC), Just the Ten of Us (ABC) and Day By Day (NBC); 55. Falcon Crest (CBS) and

Wiseguy (CBS); 57. Hooperman (ABC); 58. Coach (ABC), Tuesday Night Movie (CBS), Monday Night Movie (CBS) and Baby Boom (NBC).

62. China Beach (ABC); 63. The Robert Guillaume Show (ABC); 64. Miami Vice (NBC) nd Coming of Age (CBS); 66. The Equalizer (CBS); 67. Paradise (CBS) and Almost Grown (CBS); 69. Beauty and the Beast (CBS) and Dynasty (ABC).

71. UNSUB (NBC) and Live-In (CBS); 73. 48 Hours (CBS); 74. The Magical World of Disney (NBC); 75. Heartland (CBS); 76. Saturday Night Movie (ABC); 77. Tour of Duty (CBS) and Tattinger's (NBC); 80. Quantum Leap (NBC) and A Man Called Hawk (ABC).

82. Incredible Sunday (ABC) and Hard Time on Planet Earth (CBS); 84. Dolphin Cove (CBS) 85. Sonny Spoon (NBC); 86. Something Is Out There (NBC); 87. Police Story (ABC); 88. The Smothers Brothers Comedy Hour (CBS); 89. Family Classics (ABC); 90. West 57th (CBS), Annie McGuire (CBS) and The Van Dyke Show (CBS).

93. The Jim Henson Hour (NBC); 94. High Risk (CBS); 95. Dirty Dancing (CBS); 96. Simon & Simon (CBS) and Knightwatch (ABC); 98. Great Circuses of the World (ABC); 99. Heartbeat (ABC); 100. Murphy's Law (ABC); 101. TV 101 (CBS); 102. Raising Miranda (CBS); 103. A Fine Romance (ABC); and 104. Studio 5B (ABC).

Find your favorite shows on the list. If they're in the top 30, you can relax - odds are they'll be back next season. If they're ranked somewhere between 30 and 50, there's a good chance they'll be back - but there's also a good chance they won't.

The chances of cancellation incease dramatically for shows ranked between 50 and 70. And the only way a show ranked below 70 will survive is if its network is unusually committed to it - which just might save "Tour of Duty," "Quantum Leap," "West 57th" and "The Jim Henson Hour."

Letters wouldn't hurt, either, although I must admit that I'm becoming less enchanted with the notion of expressing programming concerns to network executives, especially in light of the remarkable success programming campaigns aimed at advertisers have had lately. Still, it's the network guys who'll be making their decisions on the new fall season in the next few weeks, so you might want to do a little lobbying for the TV series of your choice with its network. Drop a line - kindly and gently, of course - to the entertainment president at:

NBC 3000 W. Alameda Ave.

Burbank, Calif. 91523

ABC

2040 Avenue of the Stars

Los Angeles, Calif. 90067

CBS

Television City

7800 Beverly Blvd.

Los Angeles, Calif. 90036