1-5 are the best Seasons in my opinion. I watched the first Season when it was aired on BBC 2, back when the BBC had the rights to air it before Sky. I don't rate 6-8 as highly as the first 5, but I did enjoy them. Season 6 is the weakest of the lot, and is generally considered the weakest from most die hard 24 fans - just read around the fan sites if you are unsure. Season 7 greatly improves in quite a few ways over 6, and I love the twists in that. Season 8 has its moments but was disjointed at times, a bit like 7, which is why I don't rate them as highly and 1-5, which I feel are more consistent.
Anyway, it's hard to talk about specifics without spoiling too much. Just watch them all, and make your own opinion. I'm sure you'll enjoy them. I know a lot of people who came into 24 late prefer the later Seasons for some reason, but I guess everyone has their likes and dislikes regarding the strengths and weaknesses of the show. For me, I prefer 24 not just for Jack but the strength of the characters around him.