Traditionally I've always considered my top 5 bands to be:
1. Iron Maiden
2. Blind Guardian
3. Dream Theater
4. Megadeth
5. Testament,
but recently my love for Dream Theater has cooled somewhat due to their less-than-impressive most recent album, 'The Astonishing', so I'm not entirely sure if I would still consider them to be one of my top five bands. My love for Megadeth is also not as strong as it once was as their most recent albums haven't been exactly stellar either and there has been a bit of a degradation in their live performances. In the past few years, since I've been using Spotify, my music taste has expanded quite a bit and I listen to bands who, four years ago, I never thought I'd ever be a fan of such as Delain and Epica. I've also been listening to a lot more Evergrey and Gojira than I had been a few years ago, but would I include any of these bands in my top 5? I'm not sure.
It's really hard to rank bands as I go through phases of listening to some bands more than others. For example, there are certain bands who tour a lot more than other bands so I will listen to the bands who I'm going to see more than the bands who don't tour which creates a sort of unconscious bias towards bands who tour more frequently. I suppose the five bands listed at the top would constitute my "all time" top five but there are several bands just below vying for inclusion and the "current" top 5 is a very fluid thing.