Which molecule is more acidic: CHF3 or CHBr3?
The following table contains some relevant data, the $\mathrm{p}K_\mathrm{a}$'s of the various haloforms along with the Pauling electronegativity of the corresponding halogen.

\begin{array}{|c|c|c|} \hline
\text{Haloform} &amp; \mathrm{p}K_\mathrm{a} &amp; \text{Electronegativity}\\ \hline
\ce{CHF3} &amp; 25\mathrm{-}28 &amp; 3.98\\ \hline
\ce{CHCl3} &amp; 15.7 &amp; 3.16\\ \hline
\ce{CHBr3} &amp; 13.7 &amp; 2.96\\ \hline
\ce{CHI3} &amp; \sim 25 &amp; 2.66\\ \hline
\end{array}

Looking at the $\mathrm{p}K_\mathrm{a}$'s, the first thing we notice is that the two haloforms at the beginning and end of the series are weaker acids than the two haloforms in the middle of the series. This behavior is often seen when there are at least two factors affecting acidity and they are in opposition to each other.

Let's consider inductive, steric and resonance effects and examine their expected effect upon haloform acidity.


Inductive effects should correlate with electronegativity.  Based only on inductive effects we would expect fluoroform to ionize and produce the most inductively stabilized cabanion ($\ce{CF3^{-}}$), while iodoform would ionize to produce the least inductively stabilized carbanion ($\ce{CI3^{-}}$).
Steric effects are often discussed in terms of bulky groups coming too close to one another, but what we are really saying is that the electron clouds around the groups are coming too close to one another causing electron-electron repulsion. Iodoform is a very bulky molecule.  The iodine substituents are very large and there are many lone pair-lone pair repulsions.  Upon ionizing iodoform can relax from a crowded $\mathrm{sp^3}$-like geometry to a more open and spread out $\mathrm{sp^2}$-like geometry.  As the halogen gets smaller there is less of a steric \ electron repulsion driving force for haloform ionization.
Resonance effects are not a significant stabilizing factor in the haloform series. Significant low energy resonance structures cannot be drawn. Thanks to TanYongBoon for noting this.


Conclusion: Inductive effects and steric effects (steric meaning electron-electron repulsion) control acidity in the haloform series. The two effects are in opposition to one another.  The inductive effect strongly stabilizes the fluoroform anion and plays a lesser role as we move down the series.  Steric \ electron repulsion effects strongly stabilize the iodoform anion and plays a lesser role as we move up the series.  Said differently, only one effect stabilizes the fluoroform (inductive) and iododform (steric) anions, but both inductive and steric effects play a role in stabilizing the chloroform and bromoform anions making chloroform and bromoform the most acidic molecules in the series.