I own all three, and from my experience the weakest console is the Sega Saturn, it has a very poor library, it has a few games you can enjoy, but none of which are worth buying a Sega Saturn for.
If you are going to buy any Sega Console I'd recommend either the Genesis or the Dreamcast.
The Dreamcast was designed to compete with consoles that were on the market at the time, being the N64 and PS1, but was far more powerful than both of them combined and was way ahead of it's time with VMUs and a freaking modem, It has an excellent library well worth your time. Tittles like Sonic Adventure, Crazy Taxi and Shadow Man are very fun. It's rather easy to make back ups for the console, especially if you happen to get a model 1. I'm not saying get games for free that's illegal, I'm just saying you can make backup copies of games you already own so you don't have to risk damaging them.
I grew up with the N64, I have a large library for it, many exclusives and VERY few 3rd party games, Nintendo has almost always been a 1st party company, Excluding the NES which has a massive library, for every 1 1st party exclusive there are 30 3rd party games, it has a massive library of more than 900 games including all unlicensed and homebrew games made for it.
The PS1 for me had a few games I enjoyed like Spyro, Crash bandicoot and GTA, but it was lacking overall in games I enjoyed at the time. I prefer 3d platformers, action and adventure games with an occasional fighting game and driving game.
I say ignore the saturn, and look into the N64, PS1 and Dreamcast libraries and see what you find, decide based on your preferences. I myself own all consoles including the saturn.