I need to draw a picture, the picture is showed below. Which package is better to draw it,TikZ?

I need to draw a picture, the picture is showed below. Which package is better to draw it,TikZ?

A PSTricks solution:
\documentclass{article}
\usepackage{pstricks}
\begin{document}
\begin{pspicture}(8,4)
\large
\psframe(0,0)(8,4)
\rput(0.5,0.5){$\mathbf{U}$}
\pscircle(1.8,2.8){0.7}
\rput(2.6,3.6){$\mathbf{A}$}
\pscircle(6.2,2.5){1.3}
\rput(7.5,3.6){$\mathbf{B}$}
\pscircle(5.3,1.5){1.3}
\rput(6.5,0.4){$\mathbf{C}$}
\end{pspicture}
\end{document}

Can't let a pstricks answer alone, it needs company BTW ;-)
\documentclass[tikz,border=4]{standalone}
\usetikzlibrary{fit}
\begin{document}
\begin{tikzpicture}
\node[circle,draw,inner sep=0.7cm,label={[font=\bfseries]60:A}] (a) at (1,1.5) {};5
\node[circle,draw,inner sep=1.3cm,label={[font=\bfseries]60:B}] (b) at (8.5,1.5) {};
\node[circle,draw,inner sep=1.3cm,label={[font=\bfseries]-60:C}] (c) at (7,0) {};
\node[draw,fit=(a)(b)(c),inner sep=7pt,
label={[font=\bfseries,shift={(4ex,4ex)}]south west:U}] {};
\end{tikzpicture}
\end{document}

Another TikZ option, but without the fit library and with an auxiliary style:
\documentclass[tikz,border=4]{standalone}
\begin{document}
\begin{tikzpicture}[
mcircle/.style={circle,draw,inner sep=#1}
]
\node[mcircle=0.7cm,label={60:$\mathbf{A}$}]
(a) at (1,1.5) {};5
\node[mcircle=1.3cm,label={60:$\mathbf{B}$}]
(b) at (8.5,1.5) {};
\node[mcircle=1.3cm,label={-60:$\mathbf{C}$}]
(c) at (7,0) {};
\draw
([shift={(-8pt,8pt)}]a.west|-b.north) rectangle ([shift={(8pt,-8pt)}]b.east|-c.south);
\node at (a.west|-c.south) {$\mathbf{U}$};
\end{tikzpicture}
\end{document}

Another option.
\documentclass{scrartcl}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[font=\bfseries]
\draw (4,2) rectangle (-4,-2) node[above right] {U};
\draw ( -2,0.8) circle[radius=0.7] +( 50:0.7) node[right] {A};
\draw (2.5,0.7) circle[radius=1.2] +( 45:1.2) node[right] {B};
\draw (1.5,-.7) circle[radius=1.2] +(-40:1.2) node[right] {C};
\end{tikzpicture}
\end{document}
