Fun is in the experience, not the machine. That being said, in my stable of 9, the ones that give me the biggest internal grin are :
1) Any cross bike I just finished a race on or
2) The 80's steel fixie I ride in miserable winter weather (total cost to me about $80).

I enjoy my higher end race bikes, but they're tools, not toys.

any bike that i can just grab, go for a ride on and not have to worry about.
I love my road/race bikes but they have to be cleaned and serviced now and then, My mtb is to pretty to leave dirty so for me the most fun is my 'do everything' bike' back half road bike, front half mtb built to takr racks and mudguads, it just works no matter how much abuse it gets.

Agreed. Personally, I will also add that it needs to be reasonably quiet (no creaking, pinging, cables rattling etc.).

At the moment, my steel bike is my "fun" bike, even though it is not my lightest or stiffest.

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My Bikes

My Photoblog

It seems current road bikes require too much maintainance. I avoid many things on the road or the sky and worry about cleaning after riding. Riding should be simple. Simple = fun

I actually enjoy cleaning my bike!

I don't have a bike I would describe as "Not fun". All 3 of my bikes are fun and I like them all for different reasons. They all fit differently and they all perform differently, but no question I am the weakest link!!

All my bikes are fun because riding a bike is fun.

My road bike is light, fast, and responsive. My dual-suspension MTB can ride up and over anything. My tandem is a great way to enjoy cycling with a partner.

My all-time favorite now is an old KHS Pro ST soft-tail built with Tange Ultra OX steel tubing. I converted it to a single speed long before KHS made their Solo One by using an old linear pull brake caliper mounted in the derailleur hanger as a chain tensioner. I replaced the crappy OE brakes with Avid BB7s and put semi-slick lightweight tires on it. It now has a new Marz fork to replace the tired Flylight Z4. It is a bomber bike that takes no preparations to ride because of the Azonic platform pedals. I love to just cruise around and have fun on it...

Cheers

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1984 Ciocc Aquila 84 - SLX frame and full Campy Super Record
2006 Orbea Lobular - Aluminum and Carbon frame with Ultegra