Whilst I wouldn't want to be without an FdL for combat, and a Python for everything else, I am an explorer before any other concerns so the rest of my top 5 has to be explorers.
But not the obvious three. I've been flying some different ships into the black in the last few months, each brings it's own unique enjoyment and challenges. The challenge of using non-optimal explorers has helped to keep exploration fresh, and adds to any feeling of achievement.
So, in no particular order:
1) Python - best all rounder, essential for when you need to grind missions, mining etc and still defend yourself.
2) Fer de Lance - If you have to fight, an over-powered killing machine will at least make it quick and painless (for you at least).
3) Sidewinder - Yes, that ship you dump as soon as you can afford anything else is a perfectly good explorer if you don't want to take the kitchen sink with you on a speed run. Keep it in economical, and watch your fuel when scanning an expansive system and it's fine and dandy. It is a slow-boat, use for those trips where you're aiming to scan as much as possible. Cheap as chips, until you buy an ADS.
4) Dolphin - Good jump range, rarely overheats, handles well,
lands well with tall legs and no parts hanging down to obstruct getting your srv in and out. It looks good too, and is one of the cheaper options for exploration. No surprise it's good for deep space passenger missions, with a size 5 passenger restricted slot for your cabin, and nimble enough to avoid being scanned in and out of the stations.
5) Type 10 Defender - Higher jump range than could have been reasonably expected,
enormous fuel scoop (if you use the size 8 slot, and what else would you use for, out in the black?) magisterial handling, and cockpit view to die for. Yes, I know, she turns like an ice-age and lands like a nuclear strike, but she's got character - something I find the Anaconda, for all it's advantages, lacks.
Moral of the story? Spice up your favourite activity by trying something different every now and then.