It totally depends on what your budget is, and what features you want from your handset. By making the most of these Samsung Galaxy S21 deals, you will be opting for the most affordable of the S21 range, but will not enjoy the full features of the S21 Ultra, or the extra size of the + and Ultra.
However, the S21 is no slouch when it comes to killer features, and has the same cameras as the S21+, same RAM, and same Snapdragon 888 chip and Android 11 with One UI 3.0 OS as both of its bigger brothers.
One word of note for creatives, the S21 Ultra does have S-Pen stylus compatibility, which the S21 and S21+ does not. but you'll have to buy that stylus in addition to the more expensive S21 Ultra.