Preferred "natural rivals" for MLB
 

MLB has its "natural rivals" that play every year.

There are however a couple of "split rivalries".

In those cases, rather than the same matchups every year, they alternate year-to-year. One year is...
...and the next year is...
If those split rivalries were locked into the same matchups each year, which combination would you prefer?

It is worth noting that there really only appear to be two locked-in matchups in the wests. Designated rivals by year since this schedule format was introduced (the 15/15 era, as it were):
It did look like D'Backs/Astros was going to be a permanent thing, but they aren't playing this year.

In the east, for whatever reason the pairing of Boston/Atlanta and Toronto/Philadelphia just feels better IMO. I guess it's putting the historical Boston pair together and the '93 WS rematch feels good too.

For the Easts:

BOS/ATL: The historical Boston pair.
TOR/PHI: The leftovers. Interestingly, either option gives a World Series rematch.

For the Wests:

COL/HOU: HOU gets a team they were actually in the NL West with, though only for a year.
SD/SEA: The Pacific time teams, who were considered natural rivals until 2015.
ARI/TEX: The leftovers.

As a sidenote, here are the historical East rivals. Keep in mind that TOR had an actual rival through 2005:

In 2003, ATL played one series against each of BOS and TBD, while PHI played one series against each of BAL and BOS.
In 2004 and 2005, both ATL and PHI played one series against each of BAL and BOS.
In 2006, ATL played the same number of games against every AL East team.
In 2008, PHI played one series against each of BOS and TOR.
In 2011, ATL played one series against each of BAL and TOR, while PHI played one series against each of BOS and TOR.
In 2012, PHI played the same number of games against every AL East team, except NYY who they did not play.