IGT S+ is my opinion.
Cheap Parts, replace the battery every 15years or so.
Not a lot to go wrong with the machine.
Lots of game kits around, glass is the same size as the S2000, so lots of glass kits/strips around so you can change up the themes.
The 27c256 chips are still widely available if your into burning your own eproms - for backup purposes of course.
On the downside - bill validators are not getting any updates for the new bills. If that is a factor then a S2000 would probably be the next best thing.
The S2000 is a bit more complicated machine, and burning sims requires a fair bit more expertise and hardware. it can run all the S+ software.
With an S2000 - You can use TITO and a printer instead of a coin hopper. Printer paper needs refreshing and as its heat imaging any paper you have will eventually turn black by itself so you will have future supply challenges.