Keeping it down to 5 is almost impossible, but I'll try.
2000AD - 3 classic strips IMHO are Invasion! M.A.C.H. One and Meltdown Man. Loads more classics in 2000AD but these three are the ones that I will never tire of reading. If I have to pick one, then it is unsurprisingly Invasion! especially when you consider my avatar.
Hotspur - A mixed bag with lots of instantly forgettable strips but Danny Boyd, Flying Fury, The Deathless Men and The Black Sapper are the four that stand out most to me. Winner of this lot is Flying Fury.
Victor - Enjoyed it immensely, but not one of the strips has that magic something to make me go Oh yes!
Warlord - Ooh, now here it get tough. UJJ, Code Name Warlord, The Secret Files Of The Luftwaffe, Killer Kane (by Ian Kennedy of course!

), Kampgruffe Falken, Iron Annie, Sgt Rayker, Holocaust Squadron and Big Willi (no jokes please, I can do double-entendres without anyone's help! For me, the best one is Big Willi. It had a great mix of action and black humour. That's surprised me as growing up, my favourite was Holocaust Squadron and it was the one that I most looked forward to re-reading.
Starlord - Desperately wanted to pick one from here as they were all good but none quite make it into my top 5. Bang goes my comic credentials.
Action - Now another tough one. Lots of great stuff in here with Pat Mills, John Wagner and Tom Tully all firing on all cylinders, but it suffered from us not having closure (dirty rotten censorious baskets), but after consideration Death Game 1999 makes it into my top 5. Great art and being inspired by one of my all-time favourite films didn't hurt either!
Eeek, that leaves me only one choice from Megazine, Tiger, Scorcher, Bullet, Crunch, Roy Of The Rovers, Battle ( I could go on, but I won't!)
Last one into my top 5 has to be Angry Planet from Tornado. That just blew me away as a kid and it still looks top-notch almost 30 years later.
I know that I have completely overlooked the funnies and the girls comics, but we are all different and have different tastes. I know that I desperately want to put more in but I'm trying hard not to go over the 5 story limit.