Question: I accidentally left my tefillin at work. Should I daven with a minyan without my tefillin and put them on later, or daven with my tefillin without a minyan?
Answer: The Gemara (Berachos 14b) teaches that one must wear one’s tefillin while reciting the shema and amida. One who recites the shema without wearing his tefillin is considered to have given false testimony about himself (See Mishna Berura 25:14). Likewise, reciting the amida is part of the complete acceptance of the mitzvos.
The Mishna Berura (ibid.; 46:33) writes that one who does not have their tefillin must still recite the shema. Wearing tefillin and reciting the shema are independent mitzvos, and one only transgresses when they deliberately avoid wearing their tefillin.
The Magen Avraham (66:12) questions whether one who does not have their tefillin in time to daven with a minyan should wait to daven with their tefillin. He concludes that it is more important to daven with one’s tefillin. The Shulchan Aruch Harav (OC 66:11), Mishna Berura (66:40) and Kaf Hachaim (OC 25:28) concur, though they each stress that this only applies if one will not end up davening too late as a result (See Minchas Yitzchak 2:107:4).
In conclusion, one who has a choice between davening in a minyan without their tefillin, or davening alone while wearing them, should rather daven alone. However, they must ensure that they don’t recite the shema and amida too late.